fs: Silence GCC signed/unsigned warnings
fs: Silence GCC signed/unsigned warnings

The SDFileSystem class contained a few routines which compared a signed
integer loop index to an unsigned integer length/size.  I switched the
loop index to be uint32_t as well.

nRFMesh/mbed-os,fpiot/mbed-ats,Timmmm/mbed,svastm/mbed,wodji/mbed,radhika-raghavendran/mbed-os5.1-onsemi,ryankurte/mbed-os,RonEld/mbed,maximmbed/mbed,svastm/mbed,fpiot/mbed-ats,netzimme/mbed-os,larks/mbed,nvlsianpu/mbed,getopenmono/mbed,Sweet-Peas/mbed,pbrook/mbed,pbrook/mbed,devanlai/mbed,infinnovation/mbed-os,nRFMesh/mbed-os,NordicSemiconductor/mbed,mnlipp/mbed,autopulated/mbed,c1728p9/mbed-os,sam-geek/mbed,iriark01/mbed-drivers,mikaleppanen/mbed-os,theotherjimmy/mbed,CalSol/mbed,RonEld/mbed,Shengliang/mbed,NitinBhaskar/mbed,tung7970/mbed-os,K4zuki/mbed,nRFMesh/mbed-os,mikaleppanen/mbed-os,bremoran/mbed-drivers,struempelix/mbed,radhika-raghavendran/mbed-os5.1-onsemi,svastm/mbed,Marcomissyou/mbed,CalSol/mbed,mnlipp/mbed,EmuxEvans/mbed,andcor02/mbed-os,screamerbg/mbed,bulislaw/mbed-os,GustavWi/mbed,dbestm/mbed,netzimme/mbed-os,monkiineko/mbed-os,jeremybrodt/mbed,FranklyDev/mbed,jrjang/mbed,bulislaw/mbed-os,geky/mbed,mmorenobarm/mbed-os,Tiryoh/mbed,struempelix/mbed,kpurusho/mbed,bentwire/mbed,EmuxEvans/mbed,brstew/MBED-BUILD,maximmbed/mbed,K4zuki/mbed,pedromes/mbed,cvtsi2sd/mbed-os,brstew/MBED-BUILD,K4zuki/mbed,bikeNomad/mbed,svastm/mbed,logost/mbed,HeadsUpDisplayInc/mbed,alertby/mbed,jrjang/mbed,betzw/mbed-os,rosterloh/mbed,mbedmicro/mbed,screamerbg/mbed,Shengliang/mbed,mnlipp/mbed,Sweet-Peas/mbed,hwfwgrp/mbed,nabilbendafi/mbed,jferreir/mbed,GustavWi/mbed,Archcady/mbed-os,masaohamanaka/mbed,jrjang/mbed,adustm/mbed,mazimkhan/mbed-os,nabilbendafi/mbed,pradeep-gr/mbed-os5-onsemi,mnlipp/mbed,adamgreen/mbed,bcostm/mbed-os,jeremybrodt/mbed,Tiryoh/mbed,JasonHow44/mbed,bikeNomad/mbed,sam-geek/mbed,ryankurte/mbed-os,Shengliang/mbed,svogl/mbed-os,andreaslarssonublox/mbed,geky/mbed,CalSol/mbed,j-greffe/mbed-os,adamgreen/mbed,xcrespo/mbed,betzw/mbed-os,svogl/mbed-os,fanghuaqi/mbed,wodji/mbed,j-greffe/mbed-os,Shengliang/mbed,fpiot/mbed-ats,hwfwgrp/mbed,j-greffe/mbed-os,kpurusho/mbed,DanKupiniak/mbed,sam-geek/mbed,alertby/mbed,tung7970/mbed-os-1,fanghuaqi/mbed,screamerbg/mbed,HeadsUpDisplayInc/mbed,mmorenobarm/mbed-os,mmorenobarm/mbed-os,screamerbg/mbed,theotherjimmy/mbed,Tiryoh/mbed,iriark01/mbed-drivers,0xc0170/mbed-drivers,pedromes/mbed,mikaleppanen/mbed-os,kpurusho/mbed,pi19404/mbed,logost/mbed,geky/mbed,0xc0170/mbed-drivers,radhika-raghavendran/mbed-os5.1-onsemi,JasonHow44/mbed,jeremybrodt/mbed,mnlipp/mbed,bcostm/mbed-os,pradeep-gr/mbed-os5-onsemi,infinnovation/mbed-os,netzimme/mbed-os,autopulated/mbed,mmorenobarm/mbed-os,jferreir/mbed,fpiot/mbed-ats,j-greffe/mbed-os,bentwire/mbed,pedromes/mbed,EmuxEvans/mbed,CalSol/mbed,betzw/mbed-os,K4zuki/mbed,sg-/mbed-drivers,monkiineko/mbed-os,kpurusho/mbed,pradeep-gr/mbed-os5-onsemi,ryankurte/mbed-os,dbestm/mbed,nRFMesh/mbed-os,nabilbendafi/mbed,cvtsi2sd/mbed-os,betzw/mbed-os,c1728p9/mbed-os,dbestm/mbed,mikaleppanen/mbed-os,JasonHow44/mbed,getopenmono/mbed,c1728p9/mbed-os,fpiot/mbed-ats,brstew/MBED-BUILD,arostm/mbed-os,YarivCol/mbed-os,sam-geek/mbed,adustm/mbed,masaohamanaka/mbed,fvincenzo/mbed-os,sam-geek/mbed,Archcady/mbed-os,bentwire/mbed,rgrover/mbed,Shengliang/mbed,jamesadevine/mbed,fvincenzo/mbed-os,kpurusho/mbed,jferreir/mbed,NXPmicro/mbed,theotherjimmy/mbed,bentwire/mbed,GustavWi/mbed,K4zuki/mbed,jamesadevine/mbed,Tiryoh/mbed,NXPmicro/mbed,geky/mbed,bulislaw/mbed-os,ban4jp/mbed,FranklyDev/mbed,hwfwgrp/mbed,pi19404/mbed,kl-cruz/mbed-os,tung7970/mbed-os,autopulated/mbed,al177/mbed,karsev/mbed-os,NXPmicro/mbed,betzw/mbed-os,karsev/mbed-os,naves-thiago/mbed-midi,alertby/mbed,svogl/mbed-os,bcostm/mbed-os,infinnovation/mbed-os,getopenmono/mbed,EmuxEvans/mbed,ban4jp/mbed,FranklyDev/mbed,ARM-software/mbed-beetle,nRFMesh/mbed-os,xcrespo/mbed,andreaslarssonublox/mbed,mbedmicro/mbed,NordicSemiconductor/mbed,jamesadevine/mbed,Timmmm/mbed,jrjang/mbed,arostm/mbed-os,jpbrucker/mbed,karsev/mbed-os,mazimkhan/mbed-os,tung7970/mbed-os,netzimme/mbed-os,nabilbendafi/mbed,pi19404/mbed,xcrespo/mbed,masaohamanaka/mbed,infinnovation/mbed-os,HeadsUpDisplayInc/mbed,jpbrucker/mbed,betzw/mbed-os,Timmmm/mbed,FranklyDev/mbed,naves-thiago/mbed-midi,monkiineko/mbed-os,svogl/mbed-os,ban4jp/mbed,bikeNomad/mbed,NitinBhaskar/mbed,struempelix/mbed,bikeNomad/mbed,mbedmicro/mbed,ryankurte/mbed-os,jferreir/mbed,pedromes/mbed,ban4jp/mbed,alertby/mbed,monkiineko/mbed-os,c1728p9/mbed-os,kl-cruz/mbed-os,andcor02/mbed-os,adamgreen/mbed,GustavWi/mbed,wodji/mbed,adamgreen/mbed,fahhem/mbed-os,tung7970/mbed-os-1,HeadsUpDisplayInc/mbed,adustm/mbed,NitinBhaskar/mbed,netzimme/mbed-os,bulislaw/mbed-os,cvtsi2sd/mbed-os,devanlai/mbed,maximmbed/mbed,bcostm/mbed-os,arostm/mbed-os,jpbrucker/mbed,netzimme/mbed-os,bremoran/mbed-drivers,devanlai/mbed,pradeep-gr/mbed-os5-onsemi,fvincenzo/mbed-os,xcrespo/mbed,catiedev/mbed-os,rosterloh/mbed,kl-cruz/mbed-os,sg-/mbed-drivers,andcor02/mbed-os,pedromes/mbed,jpbrucker/mbed,cvtsi2sd/mbed-os,Willem23/mbed,tung7970/mbed-os-1,catiedev/mbed-os,cvtsi2sd/mbed-os,bcostm/mbed-os,pi19404/mbed,radhika-raghavendran/mbed-os5.1-onsemi,maximmbed/mbed,ryankurte/mbed-os,jeremybrodt/mbed,j-greffe/mbed-os,cvtsi2sd/mbed-os,autopulated/mbed,Willem23/mbed,Willem23/mbed,kjbracey-arm/mbed,bikeNomad/mbed,arostm/mbed-os,svogl/mbed-os,NXPmicro/mbed,andcor02/mbed-os,mazimkhan/mbed-os,geky/mbed,autopulated/mbed,alertby/mbed,fanghuaqi/mbed,ARM-software/mbed-beetle,GustavWi/mbed,monkiineko/mbed-os,mmorenobarm/mbed-os,maximmbed/mbed,tung7970/mbed-os,catiedev/mbed-os,getopenmono/mbed,adustm/mbed,EmuxEvans/mbed,bcostm/mbed-os,Sweet-Peas/mbed,al177/mbed,logost/mbed,fahhem/mbed-os,adamgreen/mbed,al177/mbed,ARM-software/mbed-beetle,Timmmm/mbed,autopulated/mbed,jferreir/mbed,Willem23/mbed,HeadsUpDisplayInc/mbed,mazimkhan/mbed-os,fpiot/mbed-ats,al177/mbed,theotherjimmy/mbed,mbedmicro/mbed,mikaleppanen/mbed-os,c1728p9/mbed-os,pradeep-gr/mbed-os5-onsemi,radhika-raghavendran/mbed-os5.1-onsemi,rgrover/mbed,larks/mbed,rosterloh/mbed,EmuxEvans/mbed,fahhem/mbed-os,fahhem/mbed-os,CalSol/mbed,alertby/mbed,hwfwgrp/mbed,pedromes/mbed,nvlsianpu/mbed,radhika-raghavendran/mbed-os5.1-onsemi,Archcady/mbed-os,c1728p9/mbed-os,larks/mbed,larks/mbed,mazimkhan/mbed-os,Marcomissyou/mbed,Timmmm/mbed,devanlai/mbed,al177/mbed,brstew/MBED-BUILD,andreaslarssonublox/mbed,xcrespo/mbed,DanKupiniak/mbed,naves-thiago/mbed-midi,hwfwgrp/mbed,YarivCol/mbed-os,dbestm/mbed,getopenmono/mbed,Timmmm/mbed,fvincenzo/mbed-os,RonEld/mbed,monkiineko/mbed-os,larks/mbed,arostm/mbed-os,fahhem/mbed-os,jpbrucker/mbed,devanlai/mbed,jamesadevine/mbed,theotherjimmy/mbed,pbrook/mbed,andreaslarssonublox/mbed,larks/mbed,hwfwgrp/mbed,nabilbendafi/mbed,pbrook/mbed,pbrook/mbed,nvlsianpu/mbed,rosterloh/mbed,pradeep-gr/mbed-os5-onsemi,wodji/mbed,jpbrucker/mbed,rosterloh/mbed,kjbracey-arm/mbed,mikaleppanen/mbed-os,tung7970/mbed-os-1,struempelix/mbed,kjbracey-arm/mbed,svastm/mbed,jferreir/mbed,NordicSemiconductor/mbed,YarivCol/mbed-os,nvlsianpu/mbed,fahhem/mbed-os,naves-thiago/mbed-midi,andreaslarssonublox/mbed,pbrook/mbed,catiedev/mbed-os,Sweet-Peas/mbed,Archcady/mbed-os,naves-thiago/mbed-midi,bulislaw/mbed-os,Willem23/mbed,RonEld/mbed,wodji/mbed,infinnovation/mbed-os,brstew/MBED-BUILD,nvlsianpu/mbed,FranklyDev/mbed,NordicSemiconductor/mbed,rosterloh/mbed,nRFMesh/mbed-os,Tiryoh/mbed,wodji/mbed,CalSol/mbed,bentwire/mbed,logost/mbed,RonEld/mbed,arostm/mbed-os,NXPmicro/mbed,andcor02/mbed-os,rgrover/mbed,jrjang/mbed,K4zuki/mbed,fvincenzo/mbed-os,Shengliang/mbed,YarivCol/mbed-os,ban4jp/mbed,karsev/mbed-os,infinnovation/mbed-os,nvlsianpu/mbed,rgrover/mbed,kl-cruz/mbed-os,kl-cruz/mbed-os,catiedev/mbed-os,Marcomissyou/mbed,jamesadevine/mbed,naves-thiago/mbed-midi,jamesadevine/mbed,ban4jp/mbed,Archcady/mbed-os,Marcomissyou/mbed,adustm/mbed,svogl/mbed-os,tung7970/mbed-os-1,masaohamanaka/mbed,Archcady/mbed-os,struempelix/mbed,al177/mbed,mnlipp/mbed,YarivCol/mbed-os,NXPmicro/mbed,Marcomissyou/mbed,screamerbg/mbed,pi19404/mbed,karsev/mbed-os,dbestm/mbed,screamerbg/mbed,kpurusho/mbed,getopenmono/mbed,jrjang/mbed,mazimkhan/mbed-os,tung7970/mbed-os,Tiryoh/mbed,mbedmicro/mbed,bentwire/mbed,struempelix/mbed,ARM-software/mbed-beetle,JasonHow44/mbed,pi19404/mbed,xcrespo/mbed,Sweet-Peas/mbed,masaohamanaka/mbed,adamgreen/mbed,brstew/MBED-BUILD,RonEld/mbed,nabilbendafi/mbed,masaohamanaka/mbed,dbestm/mbed,logost/mbed,kl-cruz/mbed-os,jeremybrodt/mbed,fanghuaqi/mbed,adustm/mbed,NitinBhaskar/mbed,karsev/mbed-os,YarivCol/mbed-os,mmorenobarm/mbed-os,DanKupiniak/mbed,NitinBhaskar/mbed,HeadsUpDisplayInc/mbed,JasonHow44/mbed,rgrover/mbed,Sweet-Peas/mbed,JasonHow44/mbed,fanghuaqi/mbed,devanlai/mbed,catiedev/mbed-os,maximmbed/mbed,theotherjimmy/mbed,logost/mbed,Marcomissyou/mbed,DanKupiniak/mbed,kjbracey-arm/mbed,j-greffe/mbed-os,bulislaw/mbed-os,andcor02/mbed-os,ryankurte/mbed-os