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 <[e-mail]>
Acked-by: Jiri Slaby <[e-mail]>
Signed-off-by: Jason Yan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/ucc_uart.c

···
  * 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)
 {
 }