[media] v4l2-ctrls: Use v4l2_subscribed_event_ops

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[hans.verkuil@cisco.com: Fix a locking bug]
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>