[media] rtl2832: wrap DVBv5 CNR to DVBv3 SNR

Change legacy DVBv3 read SNR to return values calculated by DVBv5
statistics.

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