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>

drivers/tty/serial/stm32-usart.c

@@ -600,7 +600,7 @@ static void stm32_shutdown(struct uart_port *port)
 	free_irq(port->irq, port);
 }
 
-unsigned int stm32_get_databits(struct ktermios *termios)
+static unsigned int stm32_get_databits(struct ktermios *termios)
 {
 	unsigned int bits;