stm32/timer: Use enum for indexing keyword arg in pyb_timer_init_helper.
stm32/timer: Use enum for indexing keyword arg in pyb_timer_init_helper.

pfalcon/micropython,selste/micropython,adafruit/circuitpython,pfalcon/micropython,MrSurly/micropython,selste/micropython,kerneltask/micropython,tobbad/micropython,pramasoul/micropython,henriknelson/micropython,henriknelson/micropython,trezor/micropython,pozetroninc/micropython,pramasoul/micropython,adafruit/circuitpython,pramasoul/micropython,bvernoux/micropython,selste/micropython,kerneltask/micropython,pramasoul/micropython,trezor/micropython,tobbad/micropython,pfalcon/micropython,henriknelson/micropython,pozetroninc/micropython,bvernoux/micropython,bvernoux/micropython,selste/micropython,trezor/micropython,MrSurly/micropython,trezor/micropython,bvernoux/micropython,MrSurly/micropython,kerneltask/micropython,bvernoux/micropython,pfalcon/micropython,adafruit/circuitpython,kerneltask/micropython,henriknelson/micropython,henriknelson/micropython,selste/micropython,adafruit/circuitpython,pozetroninc/micropython,kerneltask/micropython,pozetroninc/micropython,MrSurly/micropython,trezor/micropython,pozetroninc/micropython,pfalcon/micropython,adafruit/circuitpython,tobbad/micropython,MrSurly/micropython,pramasoul/micropython,tobbad/micropython,adafruit/circuitpython,tobbad/micropython