USB: serial: clean up dtr_rts

Since commit 957dacae ("TTY: fix DTR not being dropped on hang up")
dtr_rts is no longer called for uninitialised ports (e.g. after
a disconnect), which removes the need to grab the disconnect mutex.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -681,20 +681,10 @@ static int serial_port_carrier_raised(struct tty_port *port)
 static void serial_port_dtr_rts(struct tty_port *port, int on)
 {
 	struct usb_serial_port *p = container_of(port, struct usb_serial_port, port);
-	struct usb_serial *serial = p->serial;
-	struct usb_serial_driver *drv = serial->type;
+	struct usb_serial_driver *drv = p->serial->type;
 
-	if (!drv->dtr_rts)
-		return;
-	/*
-	 * Work-around bug in the tty-layer which can result in dtr_rts
-	 * being called after a disconnect (and tty_unregister_device
-	 * has returned). Remove once bug has been squashed.
-	 */
-	mutex_lock(&serial->disc_mutex);
-	if (!serial->disconnected)
+	if (drv->dtr_rts)
 		drv->dtr_rts(p, on);
-	mutex_unlock(&serial->disc_mutex);
 }
 
 static const struct tty_port_operations serial_port_ops = {