[media] hdpvr: fix querystd 'unknown format' return

If no format has been detected, then querystd should return V4L2_STD_UNKNOWN,
not V4L2_STD_ALL.

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