drivers: serial: stm32 databits depends on parity

On stm32, the M bits defines the length of the frame between the start
bit and the stop bit, eventually including the parity bit when enabled.
Fix configuration of databits to set correct M bits when parity is
enabled.
This commit tries to address issue zephyrproject-rtos/zephyr#33351

Signed-off-by: Nicolas VINCENT <nicolas.vincent@vossloh.com>
1 file changed