esp8266: Fix UART stop bit constants.
esp8266: Fix UART stop bit constants.

As per the "ESP8266 Technical Reference".

swegener/micropython,blazewicz/micropython,micropython/micropython-esp32,MrSurly/micropython,tralamazza/micropython,blazewicz/micropython,MrSurly/micropython,dmazzella/micropython,infinnovation/micropython,AriZuu/micropython,MrSurly/micropython-esp32,Timmenem/micropython,Timmenem/micropython,AriZuu/micropython,lowRISC/micropython,blazewicz/micropython,adafruit/circuitpython,Timmenem/micropython,adafruit/micropython,ryannathans/micropython,adafruit/micropython,adafruit/circuitpython,tobbad/micropython,adafruit/micropython,tralamazza/micropython,blazewicz/micropython,infinnovation/micropython,bvernoux/micropython,blazewicz/micropython,pramasoul/micropython,micropython/micropython-esp32,swegener/micropython,kerneltask/micropython,MrSurly/micropython-esp32,oopy/micropython,tobbad/micropython,AriZuu/micropython,oopy/micropython,oopy/micropython,micropython/micropython-esp32,tobbad/micropython,bvernoux/micropython,lowRISC/micropython,lowRISC/micropython,kerneltask/micropython,MrSurly/micropython,ryannathans/micropython,pramasoul/micropython,micropython/micropython-esp32,Timmenem/micropython,pozetroninc/micropython,trezor/micropython,kerneltask/micropython,MrSurly/micropython-esp32,pozetroninc/micropython,pramasoul/micropython,adafruit/micropython,henriknelson/micropython,AriZuu/micropython,trezor/micropython,trezor/micropython,lowRISC/micropython,dmazzella/micropython,kerneltask/micropython,torwag/micropython,lowRISC/micropython,dmazzella/micropython,adafruit/circuitpython,pramasoul/micropython,pfalcon/micropython,tralamazza/micropython,selste/micropython,torwag/micropython,selste/micropython,pozetroninc/micropython,adafruit/circuitpython,henriknelson/micropython,ryannathans/micropython,MrSurly/micropython,bvernoux/micropython,AriZuu/micropython,oopy/micropython,swegener/micropython,tralamazza/micropython,pramasoul/micropython,adafruit/micropython,torwag/micropython,selste/micropython,MrSurly/micropython-esp32,swegener/micropython,MrSurly/micropython-esp32,dmazzella/micropython,infinnovation/micropython,pfalcon/micropython,trezor/micropython,Timmenem/micropython,oopy/micropython,pfalcon/micropython,micropython/micropython-esp32,kerneltask/micropython,torwag/micropython,infinnovation/micropython,infinnovation/micropython,ryannathans/micropython,tobbad/micropython,pozetroninc/micropython,torwag/micropython,henriknelson/micropython,pfalcon/micropython,selste/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,ryannathans/micropython,trezor/micropython,henriknelson/micropython,pfalcon/micropython,tobbad/micropython,swegener/micropython,pozetroninc/micropython,MrSurly/micropython,bvernoux/micropython,selste/micropython