cpu/stm32_common: use correct data bits macro
cpu/stm32_common: use correct data bits macro

USART_CR1_M combines both USART_CR1_M0 and USART_CR1_M1 macros
affecting bits 12 and 28 on 7 data bits capable UARTs. Whereas
for other UARTs USART_CR1_M macro affects only bit 12.

This patch fixes wrong data bits usage on 7 data bits capable
UARTs with using USART_CR1_M0 macro for modes 8-E-x and 8-O-x.

It also simplifies bits unsetting as USART_CR1_M macro clears
all data bits related bits for both UART types.

Signed-off-by: Yegor Yefremov <379041fa42359f10443de1f461b1b247ec449fae@googlemail.com>

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