V4L/DVB (6209): pvrusb2: Better discriminate among device types

This is a bunch of cleanup in various places to improve behavior based
on actual device type being driven.  While this doesn't actually
affect operation with existing devices, it cleans things up so that it
will be easier / more deterministic when other devices are added.
Ideally we should make stuff like this table-driven, but for now this
is just a series of small incremental (read: safe) improvements.

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