[media] pvrusb2: Base available video standards on what hardware supports

With the transition to ioctl2, the pvrusb2 driver's own standards
enumeration is no longer used.  Instead a generic algorithm internal
to v4l is used (which is a great idea - since the pvrusb2
implementation itself was generic anyway).  This change ensures that
the v4l algorithm works with the correct set of hardware supported
standards.  This resolves a FIXME left behind from the videodev_ioctl2
transition.

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