[media] af9033: fix AF9033 DVBv3 signal strength measurement

Previous patch changes used signal strength firmware register from
0x800048 to 0x80004a in case of AF9033/AF9035 chip. In practice
reported values were running upside-down, when RR strength increases
reported value decreases and vice versa. That is because of 0x80004a
returns values that are dBm scale, but negative RF strength dBm
returned as positive number.

0x800048 returns 0-100, like percentage
0x80004a returns 0-255 dBm, without a negative sign

So restore old measurement now.

Cc: 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>