[media] cxd2841er: Reading SNR for DVB-C added

Now driver returns correct values for DVB-C:
 SNR (in dB)

[mchehab@s-opensource.com: fix a new function to be static]
Signed-off-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>