[media] pvrusb2: drop g/s_register ioctls

Register access to subdevices no longer needs bridge support for those
ioctls. The v4l2 core handles that these days.

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