stm32/stm32_it: Guard UART7_IRQHandler with check for UART7 define.
stm32/stm32_it: Guard UART7_IRQHandler with check for UART7 define.

All STM32 with a UART7 also have a UART8 and vice versa, but this change
improves readability and allows for them to be independent in the future.

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