stmhal: Fix USB CDC not flushing packets when an exact multiple of 64.
stmhal: Fix USB CDC not flushing packets when an exact multiple of 64.

Need to send a zero-sized packet after sending an exact multiple of 64
bytes (not just after sending 64 bytes exactly).

Addresses issue #494, part 2.

galenhz/micropython,hiway/micropython,rubencabrera/micropython,ganshun666/micropython,adafruit/circuitpython,utopiaprince/micropython,noahwilliamsson/micropython,utopiaprince/micropython,dmazzella/micropython,chrisdearman/micropython,orionrobots/micropython,Vogtinator/micropython,aethaniel/micropython,adafruit/circuitpython,cwyark/micropython,rubencabrera/micropython,martinribelotta/micropython,deshipu/micropython,cnoviello/micropython,lbattraw/micropython,kerneltask/micropython,alex-robbins/micropython,heisewangluo/micropython,pozetroninc/micropython,kerneltask/micropython,hiway/micropython,oopy/micropython,henriknelson/micropython,pozetroninc/micropython,galenhz/micropython,xyb/micropython,torwag/micropython,trezor/micropython,ryannathans/micropython,matthewelse/micropython,vitiral/micropython,HenrikSolver/micropython,xuxiaoxin/micropython,aethaniel/micropython,pramasoul/micropython,redbear/micropython,misterdanb/micropython,xyb/micropython,MrSurly/micropython-esp32,mpalomer/micropython,infinnovation/micropython,MrSurly/micropython-esp32,selste/micropython,mhoffma/micropython,mpalomer/micropython,warner83/micropython,adafruit/circuitpython,HenrikSolver/micropython,praemdonck/micropython,noahwilliamsson/micropython,Peetz0r/micropython-esp32,paul-xxx/micropython,tralamazza/micropython,blmorris/micropython,tdautc19841202/micropython,Timmenem/micropython,chrisdearman/micropython,xyb/micropython,warner83/micropython,micropython/micropython-esp32,MrSurly/micropython-esp32,tralamazza/micropython,ryannathans/micropython,xhat/micropython,mgyenik/micropython,chrisdearman/micropython,alex-march/micropython,torwag/micropython,kerneltask/micropython,noahwilliamsson/micropython,cnoviello/micropython,neilh10/micropython,supergis/micropython,bvernoux/micropython,SungEun-Steve-Kim/test-mp,puuu/micropython,bvernoux/micropython,cnoviello/micropython,turbinenreiter/micropython,PappaPeppar/micropython,alex-march/micropython,danicampora/micropython,feilongfl/micropython,emfcamp/micropython,danicampora/micropython,danicampora/micropython,slzatz/micropython,jimkmc/micropython,utopiaprince/micropython,redbear/micropython,hiway/micropython,jlillest/micropython,dhylands/micropython,pramasoul/micropython,KISSMonX/micropython,cloudformdesign/micropython,slzatz/micropython,lbattraw/micropython,jmarcelino/pycom-micropython,AriZuu/micropython,slzatz/micropython,firstval/micropython,cwyark/micropython,mpalomer/micropython,HenrikSolver/micropython,xuxiaoxin/micropython,heisewangluo/micropython,infinnovation/micropython,chrisdearman/micropython,noahchense/micropython,swegener/micropython,vriera/micropython,paul-xxx/micropython,dxxb/micropython,kostyll/micropython,ganshun666/micropython,xhat/micropython,neilh10/micropython,MrSurly/micropython-esp32,blmorris/micropython,toolmacher/micropython,jlillest/micropython,tobbad/micropython,ernesto-g/micropython,warner83/micropython,dmazzella/micropython,martinribelotta/micropython,MrSurly/micropython,firstval/micropython,dhylands/micropython,slzatz/micropython,ruffy91/micropython,ericsnowcurrently/micropython,jimkmc/micropython,pfalcon/micropython,Vogtinator/micropython,blazewicz/micropython,stonegithubs/micropython,redbear/micropython,SungEun-Steve-Kim/test-mp,bvernoux/micropython,hiway/micropython,adafruit/circuitpython,orionrobots/micropython,toolmacher/micropython,henriknelson/micropython,turbinenreiter/micropython,misterdanb/micropython,neilh10/micropython,ceramos/micropython,pramasoul/micropython,ChuckM/micropython,warner83/micropython,micropython/micropython-esp32,toolmacher/micropython,MrSurly/micropython,selste/micropython,feilongfl/micropython,suda/micropython,selste/micropython,henriknelson/micropython,trezor/micropython,pozetroninc/micropython,trezor/micropython,feilongfl/micropython,SungEun-Steve-Kim/test-mp,tuc-osg/micropython,xyb/micropython,adafruit/micropython,TDAbboud/micropython,dxxb/micropython,redbear/micropython,omtinez/micropython,ericsnowcurrently/micropython,MrSurly/micropython,dxxb/micropython,EcmaXp/micropython,vitiral/micropython,aethaniel/micropython,skybird6672/micropython,lbattraw/micropython,galenhz/micropython,jimkmc/micropython,trezor/micropython,pramasoul/micropython,lowRISC/micropython,methoxid/micropystat,praemdonck/micropython,jimkmc/micropython,tobbad/micropython,oopy/micropython,adafruit/micropython,deshipu/micropython,xuxiaoxin/micropython,kostyll/micropython,ernesto-g/micropython,vriera/micropython,lowRISC/micropython,SHA2017-badge/micropython-esp32,xhat/micropython,methoxid/micropystat,martinribelotta/micropython,mianos/micropython,turbinenreiter/micropython,xyb/micropython,kostyll/micropython,jmarcelino/pycom-micropython,suda/micropython,pozetroninc/micropython,stonegithubs/micropython,hosaka/micropython,alex-robbins/micropython,EcmaXp/micropython,alex-robbins/micropython,cloudformdesign/micropython,feilongfl/micropython,dxxb/micropython,mianos/micropython,slzatz/micropython,orionrobots/micropython,danicampora/micropython,alex-robbins/micropython,aethaniel/micropython,noahchense/micropython,AriZuu/micropython,chrisdearman/micropython,infinnovation/micropython,skybird6672/micropython,jmarcelino/pycom-micropython,vriera/micropython,oopy/micropython,blmorris/micropython,PappaPeppar/micropython,vitiral/micropython,Vogtinator/micropython,hosaka/micropython,ceramos/micropython,pfalcon/micropython,deshipu/micropython,noahchense/micropython,ChuckM/micropython,heisewangluo/micropython,deshipu/micropython,jmarcelino/pycom-micropython,adafruit/circuitpython,suda/micropython,alex-robbins/micropython,martinribelotta/micropython,paul-xxx/micropython,redbear/micropython,infinnovation/micropython,lowRISC/micropython,mgyenik/micropython,kostyll/micropython,turbinenreiter/micropython,ruffy91/micropython,infinnovation/micropython,tuc-osg/micropython,Timmenem/micropython,noahchense/micropython,methoxid/micropystat,aethaniel/micropython,KISSMonX/micropython,kerneltask/micropython,PappaPeppar/micropython,vriera/micropython,ericsnowcurrently/micropython,Vogtinator/micropython,SHA2017-badge/micropython-esp32,matthewelse/micropython,ryannathans/micropython,hosaka/micropython,blazewicz/micropython,rubencabrera/micropython,MrSurly/micropython,blmorris/micropython,mpalomer/micropython,cwyark/micropython,AriZuu/micropython,firstval/micropython,drrk/micropython,deshipu/micropython,cloudformdesign/micropython,neilh10/micropython,tobbad/micropython,mianos/micropython,SHA2017-badge/micropython-esp32,noahwilliamsson/micropython,dinau/micropython,ahotam/micropython,AriZuu/micropython,suda/micropython,feilongfl/micropython,ganshun666/micropython,puuu/micropython,mhoffma/micropython,noahwilliamsson/micropython,blazewicz/micropython,pfalcon/micropython,firstval/micropython,stonegithubs/micropython,SHA2017-badge/micropython-esp32,tdautc19841202/micropython,blazewicz/micropython,orionrobots/micropython,ernesto-g/micropython,paul-xxx/micropython,cwyark/micropython,tralamazza/micropython,ahotam/micropython,pozetroninc/micropython,danicampora/micropython,micropython/micropython-esp32,Peetz0r/micropython-esp32,swegener/micropython,lbattraw/micropython,alex-march/micropython,adafruit/circuitpython,ChuckM/micropython,pfalcon/micropython,supergis/micropython,ericsnowcurrently/micropython,ChuckM/micropython,swegener/micropython,adamkh/micropython,tobbad/micropython,Peetz0r/micropython-esp32,SungEun-Steve-Kim/test-mp,vitiral/micropython,xhat/micropython,methoxid/micropystat,TDAbboud/micropython,emfcamp/micropython,xuxiaoxin/micropython,ruffy91/micropython,omtinez/micropython,ruffy91/micropython,jlillest/micropython,neilh10/micropython,blmorris/micropython,paul-xxx/micropython,misterdanb/micropython,dhylands/micropython,tdautc19841202/micropython,adafruit/micropython,oopy/micropython,ganshun666/micropython,praemdonck/micropython,adamkh/micropython,HenrikSolver/micropython,warner83/micropython,ceramos/micropython,ernesto-g/micropython,TDAbboud/micropython,EcmaXp/micropython,jmarcelino/pycom-micropython,lowRISC/micropython,heisewangluo/micropython,hosaka/micropython,KISSMonX/micropython,rubencabrera/micropython,selste/micropython,puuu/micropython,kostyll/micropython,mianos/micropython,ganshun666/micropython,omtinez/micropython,dmazzella/micropython,henriknelson/micropython,stonegithubs/micropython,tdautc19841202/micropython,ryannathans/micropython,torwag/micropython,ahotam/micropython,ahotam/micropython,dxxb/micropython,dhylands/micropython,vriera/micropython,swegener/micropython,tobbad/micropython,kerneltask/micropython,tuc-osg/micropython,turbinenreiter/micropython,drrk/micropython,ahotam/micropython,mhoffma/micropython,Timmenem/micropython,jimkmc/micropython,mgyenik/micropython,jlillest/micropython,dinau/micropython,pramasoul/micropython,bvernoux/micropython,drrk/micropython,PappaPeppar/micropython,dhylands/micropython,dmazzella/micropython,emfcamp/micropython,toolmacher/micropython,tuc-osg/micropython,jlillest/micropython,micropython/micropython-esp32,omtinez/micropython,vitiral/micropython,henriknelson/micropython,matthewelse/micropython,PappaPeppar/micropython,methoxid/micropystat,ericsnowcurrently/micropython,emfcamp/micropython,dinau/micropython,mgyenik/micropython,emfcamp/micropython,cwyark/micropython,SHA2017-badge/micropython-esp32,suda/micropython,pfalcon/micropython,adamkh/micropython,xhat/micropython,omtinez/micropython,supergis/micropython,cloudformdesign/micropython,HenrikSolver/micropython,skybird6672/micropython,mhoffma/micropython,martinribelotta/micropython,ceramos/micropython,skybird6672/micropython,EcmaXp/micropython,tuc-osg/micropython,tralamazza/micropython,lowRISC/micropython,hiway/micropython,adamkh/micropython,SungEun-Steve-Kim/test-mp,dinau/micropython,Timmenem/micropython,supergis/micropython,trezor/micropython,alex-march/micropython,Timmenem/micropython,ernesto-g/micropython,cnoviello/micropython,TDAbboud/micropython,praemdonck/micropython,hosaka/micropython,alex-march/micropython,mpalomer/micropython,oopy/micropython,swegener/micropython,skybird6672/micropython,adamkh/micropython,blazewicz/micropython,toolmacher/micropython,drrk/micropython,torwag/micropython,KISSMonX/micropython,ryannathans/micropython,MrSurly/micropython-esp32,ceramos/micropython,AriZuu/micropython,mianos/micropython,utopiaprince/micropython,adafruit/micropython,misterdanb/micropython,xuxiaoxin/micropython,Peetz0r/micropython-esp32,matthewelse/micropython,stonegithubs/micropython,Vogtinator/micropython,rubencabrera/micropython,tdautc19841202/micropython,MrSurly/micropython,utopiaprince/micropython,orionrobots/micropython,matthewelse/micropython,misterdanb/micropython,puuu/micropython,adafruit/micropython,supergis/micropython,heisewangluo/micropython,cloudformdesign/micropython,ChuckM/micropython,bvernoux/micropython,firstval/micropython,lbattraw/micropython,noahchense/micropython,praemdonck/micropython,Peetz0r/micropython-esp32,galenhz/micropython,KISSMonX/micropython,micropython/micropython-esp32,torwag/micropython,matthewelse/micropython,dinau/micropython,drrk/micropython,TDAbboud/micropython,mhoffma/micropython,mgyenik/micropython,EcmaXp/micropython,selste/micropython,galenhz/micropython,puuu/micropython,ruffy91/micropython,cnoviello/micropython