SERIAL: omap: fix set_mctrl() breakage

c538d20c (and maybe previous commits) broke set_mctrl() by making
it only capable of setting bits in the MCR register.  This prevents
software controlled flow control and modem control line manipulation
via TIOCMSET/TIOCMBIC from working correctly.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/tty/serial/omap-serial.c

@@ -504,7 +504,7 @@ static unsigned int serial_omap_get_mctrl(struct uart_port *port)
 static void serial_omap_set_mctrl(struct uart_port *port, unsigned int mctrl)
 {
 	struct uart_omap_port *up = to_uart_omap_port(port);
-	unsigned char mcr = 0;
+	unsigned char mcr = 0, old_mcr;
 
 	dev_dbg(up->port.dev, "serial_omap_set_mctrl+%d\n", up->port.line);
 	if (mctrl & TIOCM_RTS)
@@ -519,8 +519,10 @@ static void serial_omap_set_mctrl(struct uart_port *port, unsigned int mctrl)
 		mcr |= UART_MCR_LOOP;
 
 	pm_runtime_get_sync(up->dev);
-	up->mcr = serial_in(up, UART_MCR);
-	up->mcr |= mcr;
+	old_mcr = serial_in(up, UART_MCR);
+	old_mcr &= ~(UART_MCR_LOOP | UART_MCR_OUT2 | UART_MCR_OUT1 |
+		     UART_MCR_DTR | UART_MCR_RTS);
+	up->mcr = old_mcr | mcr;
 	serial_out(up, UART_MCR, up->mcr);
 	pm_runtime_mark_last_busy(up->dev);
 	pm_runtime_put_autosuspend(up->dev);