[media] media: info leak in __media_device_enum_links()

These structs have holes and reserved struct members which aren't
cleared.  I've added a memset() so we don't leak stack information.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>