serial: core: Make enable_ms() optional

This patch makes enable_ms() optional, so we can eliminate a lot of
empty enable_ms() implementations from driver code.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1050,6 +1050,15 @@ static int uart_do_autoconfig(struct tty_struct *tty,struct uart_state *state)
 	return ret;
 }
 
+static void uart_enable_ms(struct uart_port *uport)
+{
+	/*
+	 * Force modem status interrupts on
+	 */
+	if (uport->ops->enable_ms)
+		uport->ops->enable_ms(uport);
+}
+
 /*
  * Wait for any of the 4 modem inputs (DCD,RI,DSR,CTS) to change
  * - mask passed in arg for lines of interest
@@ -1073,11 +1082,7 @@ uart_wait_modem_status(struct uart_state *state, unsigned long arg)
 	 */
 	spin_lock_irq(&uport->lock);
 	memcpy(&cprev, &uport->icount, sizeof(struct uart_icount));
-
-	/*
-	 * Force modem status interrupts on
-	 */
-	uport->ops->enable_ms(uport);
+	uart_enable_ms(uport);
 	spin_unlock_irq(&uport->lock);
 
 	add_wait_queue(&port->delta_msr_wait, &wait);
@@ -1508,7 +1513,7 @@ static int uart_carrier_raised(struct tty_port *port)
 	struct uart_port *uport = state->uart_port;
 	int mctrl;
 	spin_lock_irq(&uport->lock);
-	uport->ops->enable_ms(uport);
+	uart_enable_ms(uport);
 	mctrl = uport->ops->get_mctrl(uport);
 	spin_unlock_irq(&uport->lock);
 	if (mctrl & TIOCM_CAR)