[media] af9033: fix DVBv3 signal strength value not correct issue

Register 0x800048 is not dB measure but relative scale. Fix it and conform to NorDig specifications.

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