[media] dvb: LNA implementation changes

* use dvb property cache
* implement get (thus API minor++)
* PCTV 290e: 1=LNA ON, all the other values LNA OFF
  Also fix PCTV 290e LNA comment, it is disabled by default
Hans and Mauro proposed use of cache implementation of get as they
were planning to extend LNA usage for analog side too.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Reported-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>