[media] pvrusb2: Transform video standard detection result into read-only control ID

Other aspects of the pvrusb2 driver - including in particular those
dealing with video standards - all use internal control IDs for
uniform access by the interfaces.  By exporting the querystd result as
another control ID, uniform access to it becomes available through the
sysfs interface.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>