[media] cxd2841er: fix signal strength scale for ISDB-T

The scale for ISDB-T was wrong too: it was inverted, and
on a relative scale.

Use a linear interpolation to make it look better.
The formula was empirically determined, using 3 frequencies
(175 MHz, 410 MHz and 800 MHz), measuring from -50dBm to
-12dBm in steps of 0.5dB.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>