serial: stm32: Make stm32_get_databits static

Fix sparse warning:

drivers/tty/serial/stm32-usart.c:603:14: warning:
 symbol 'stm32_get_databits' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>