[media] v4l2-event: v4l2_event_queue: do nothing if vdev == NULL

If the vdev pointer == NULL, then just return.

This makes it easier for subdev drivers to use this function without having to
check if the sd->devnode pointer is NULL or not.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>