[media] rtl2830: wrap DVBv5 signal strength to DVBv3

Change legacy DVBv3 signal strength 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>