USB: mxuport: fix null deref when used as a console

Fix null-pointer dereference at probe when the device is used as a
console, in which case the tty argument to open will be NULL.

Fixes: ee467a1f2066 ("USB: serial: add Moxa UPORT 12XX/14XX/16XX
driver")
Cc: stable <stable@vger.kernel.org>	# v3.14
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/usb/serial/mxuport.c

@@ -1284,7 +1284,8 @@ static int mxuport_open(struct tty_struct *tty, struct usb_serial_port *port)
 	}
 
 	/* Initial port termios */
-	mxuport_set_termios(tty, port, NULL);
+	if (tty)
+		mxuport_set_termios(tty, port, NULL);
 
 	/*
 	 * TODO: use RQ_VENDOR_GET_MSR, once we know what it