V4L/DVB (11155): pvrusb2: Set up v4l2_device instance

Define a v4l2_device instance in the pvrusb2 driver and initialize /
tear it down appropriately.  This is a step in the v4l2-subdev
adoption effort.

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