media: af9013: dvbv5 signal strength

Implement dvbv5 signal strength estimate. We know tuner dependent
-80dBm and -50dBm agc values, construct line equation and use it to
map agc value to signal strength estimate.

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