V4L/DVB (11175): pvrusb2: Implement sub-device specific update framework

Lay down a foundation whereby it becomes possible to send customized
updates to specific sub-devices.  (This becomes useful for routing
configuration, which is a very sub-device specific operation.)

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