V4L/DVB (11167): pvrusb2: Tie in various v4l2 operations into the sub-device mechanism

This is another step in the v42l-subdev assimilation.  This implements
various call-outs to sub-devices based on state changes within the
pvrusb2 driver.

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