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 <[e-mail]>
Signed-off-by: YueHaibing <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/stm32-usart.c

···
 	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;