V4L/DVB (11158): pvrusb2: New device attribute mechanism to specify sub-devices

Set up new mechanism for declaring and loading appropriate sub-devices
when driver initializes.  This is another part of the v4l2-subdev
adoption.

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