[media] dib8000: make a better estimation for dBm

Use multiple linear segments to better interpolate the dBm
for the signal strength.

The table that converts from linear strength to dB was
empirically determinated with the help of a signal generator
(DTA-2111).

The entries from -35dBm to -22.5dBm were taken using just
the signal generator and the board.

For the entries from -36dBm to -51dBm, a 16 dB tap was used,
in order to extend its range.

Signals below to -51dBm are just linearly interpolated.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Patrick Boettcher <pboettcher@kernellabs.com>