[media] af9033: implement DVBv5 statistics for signal strength

Let the demod firmware estimate RF signal strength and return it
to the app as a dBm. To handle that, use thread which reads signal
strengths from firmware in 2 sec intervals when device is active.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>