[media] saa7164: fix poll bugs

- poll doesn't return negative values, so you can't return -EINVAL.
  Instead return POLLERR.
- poll can't be called if !video_is_registered(), so this test can
  be dropped.
- poll can never do a blocking wait, so remove that check.
- poll shouldn't attempt to start streaming if the caller isn't interested
  in read events.

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