stm32/uart: Use LL_USART_GetBaudRate to compute baudrate.
stm32/uart: Use LL_USART_GetBaudRate to compute baudrate.

This function includes the UART prescaler in the calculation (if it has
one, eg on H7 and WB MCUs).

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython