[media] DVB-C read signal strength added for Sony demod

cxd2841er_read_agc_gain_c added to obtain signal strength.
signal strength now relay on AGC value.

Signed-off-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>