V4L/DVB (12202): em28xx, fix lock imbalance

There is one omitted unlock in em28xx_usb_probe. Fix that.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/em28xx/em28xx-cards.c

@@ -2652,6 +2652,7 @@ static int em28xx_usb_probe(struct usb_interface *interface,
 	retval = em28xx_init_dev(&dev, udev, interface, nr);
 	if (retval) {
 		em28xx_devused &= ~(1<<dev->devno);
+		mutex_unlock(&dev->lock);
 		kfree(dev);
 		goto err;
 	}