[media] DVB: dvb_frontend: remove static assignments from dtv_property_cache_sync()

  dtv_property_cache_init().
  dtv_property_process_get().
  overwritten with invalid values, leading to partially incorrect
  results when calling FE_GET_PROPERTY.

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