[media] cx88: set dev_parent to the correct parent PCI bus

The cx88 driver has one v4l2_device, but the video nodes are owned by two
different PCI busses. So the dev_parent pointer should be set to the correct
parent bus, otherwise sysfs won't show the correct device hierarchy.
This broke starting in 3.6 after a driver change, so this patch resurrects
the correct behavior.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>