[media] pwc: Use vb2 queue mutex through a single name

This lock was being taken using two different names
(pointers) in the same function.
Both names refer to the same lock,
so this wasn't an error; but it looked very strange.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/pwc/pwc-if.c

@@ -1127,7 +1127,7 @@ static void usb_pwc_disconnect(struct usb_interface *intf)
 	v4l2_device_disconnect(&pdev->v4l2_dev);
 	video_unregister_device(&pdev->vdev);
 	mutex_unlock(&pdev->v4l2_lock);
-	mutex_unlock(pdev->vb_queue.lock);
+	mutex_unlock(&pdev->vb_queue_lock);
 
 #ifdef CONFIG_USB_PWC_INPUT_EVDEV
 	if (pdev->button_dev)