V4L/DVB (11156): pvrusb2: Changes to further isolate old i2c layer

This introduces some additional isolation in the pvrusb2 from the old
i2c layer, a step along the way to separate the driver from that layer
and to make it easier to introduce the common v4l2-subdev framework as
the eventual replacement.

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