V4L/DVB (11163): pvrusb2: Lay foundation for triggering sub-device updates

These changes set up the spot where we'll check for and set general
updates to any attached sub-devices.

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