[media] v4l: async: Make it safe to unregister unregistered notifier

Calling v4l2_async_notifier_unregister() on a notifier that hasn't been
registered leads to a crash. To simplify drivers, make it safe to
unregister a notifier that has not been registered.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/v4l2-core/v4l2-async.c

@@ -192,6 +192,9 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
 	struct device *dev[n_subdev];
 	int i = 0;
 
+	if (!notifier->v4l2_dev)
+		return;
+
 	mutex_lock(&list_lock);
 
 	list_del(&notifier->list);
@@ -225,6 +228,9 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
 		}
 		put_device(d);
 	}
+
+	notifier->v4l2_dev = NULL;
+
 	/*
 	 * Don't care about the waiting list, it is initialised and populated
 	 * upon notifier registration.