drivers/uart: stm32: Report 9bits transactions as not supported
drivers/uart: stm32: Report 9bits transactions as not supported

STM32 uart driver doesn't support 9bits transactions in any case,
so remove case were it was declared as supported.

Fixes #31799

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr