V4L/DVB (11162): pvrusb2: Tie up loose ends with v4l2-subdev setup

Tie up loose ends with v4l2-subdev setup.  Set attached module's group
ID to match our internal ID, emit a few useful messages when
sub-devices are dealt with, implement better error legs, and fix an
error in the old i2c layer (caused by changes related to the
v4l2-subdev work here).

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