[media] cx24120: Improve cooked signal strength value

Algorithm copied from cx24117.c - it seems to work. Showing 95% on
a signal I know to be somewhere around 74dBuV.

[mchehab@osg.samsung.net: merged two patches, to avoid breaking compilation]
Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>