[media] saa7164: add support for control events

Now that saa7164 uses v4l2_fh and that poll() has been fixed, it is
trivial to add support for control events.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>