[SERIAL] Don't initialise port->mctrl before calling ->startup

If a serial port is hung up using vhangup() with HUPCL cleared (so
DTR isn't dropped), and then re-opened, we don't want to drop the
DTR line.  This occurs during the login process, and dropping DTR
would cause a modem to hang up.

drivers/serial/serial_core.c

@@ -175,8 +175,6 @@ static int uart_startup(struct uart_state *state, int init_hw)
 		uart_circ_clear(&info->xmit);
 	}
 
-	port->mctrl = 0;
-
 	retval = port->ops->startup(port);
 	if (retval == 0) {
 		if (init_hw) {