V4L/DVB (9942): v4l2-dev: check for parent device in get_index.

get_index requires a valid parent device in order to discover which
indices are in use. Some drivers (e.g. pvrusb2) do not set the parent
device. In that case just return 0.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>