[media] hdpvr: allow g/s/enum/querystd when in legacy mode

Both MythTV and gstreamer expect that they can set/get/query/enumerate the
standards, even if the input is the component input for which standards
really do not apply.

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