serial: ucc_uart: make qe_uart_set_mctrl() static

This eliminates the following sparse warning:

drivers/tty/serial/ucc_uart.c:286:6: warning: symbol 'qe_uart_set_mctrl'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Acked-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200912033834.143166-1-yanaijie@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/ucc_uart.c

@@ -283,7 +283,7 @@ static unsigned int qe_uart_tx_empty(struct uart_port *port)
  * don't need that support. This function must exist, however, otherwise
  * the kernel will panic.
  */
-void qe_uart_set_mctrl(struct uart_port *port, unsigned int mctrl)
+static void qe_uart_set_mctrl(struct uart_port *port, unsigned int mctrl)
 {
 }