[media] DVB: dtv_property_cache_submit shouldn't modifiy the cache

- Use const pointers and remove assignments.
- delivery_system already gets assigned by DTV_DELIVERY_SYSTEM
  and dtv_property_cache_sync.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>