[media] au0828: fixes for timeout on no video

The first pass of fixes to cause the driver to continue streaming even when
there is no video arriving over the ITU656 bus were not adequate.  Continue
the work from the previous patch.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>