V4L/DVB (11169): pvrusb2: Note who our video decoder sub-device is, and set it up

Other code may need to treat the video decoder sub-device in a special
manner, so this change implements code to recognize when such a
sub-device is connected to the driver, does any special processing for
it, and notes who the device is for future reference.

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