serial: stm32: fix word length configuration

STM32 supports either:
- 8 and 9 bits word length (including parity bit) for stm32f4 compatible
  devices
- 7, 8 and 9 bits word length (including parity bit) for stm32f7 and
  stm32h7 compatible devices.

As a consequence STM32 supports the following termios configurations:
- CS7 with parity bit, and CS8 (with or without parity bit) for stm32f4
  compatible devices.
- CS6 with parity bit, CS7 and CS8 (with or without parity bit) for
  stm32f7 and stm32h7 compatible devices.

This patch is fixing word length by configuring correctly the SoC with
supported configurations.

Fixes: ada8618f ("serial: stm32: adding support for stm32f7")
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>