V4L/DVB: pvrusb2: Fix USB parent device reference count

pvrusb2: Correctly reference count pointer to parent USB device when linked
from sysfs interface.  This is technically a pretty nasty problem,
however as far as I know nobody had been getting burned by it (yet).

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