[media] dib8000: estimate strength in dBm

Better to have Signal strength in dB.
This takes a very rough estimation for the signal strength,
that was calibrated using a Dektec DTA-2111 Gold RF generator
and a Pixelview dib8076 stick.

It estimates the signal strength using a linear equation where:
	- the max is -22.5 dBm, with returns 55953
	- the min is -35.0 dBm, with returns 50110

With -22dBm, the signal strengh is returned as 65535.
Unfortunately, the min strength generated with DTA-2111 is
-35dBm.

It should be noticed that approximating it by a linear equation
is not right. I should probably be splitting it into 0.5 dB
linear segments, in order to get a higher precision, just like
it is done on mb86a20s, but that would force me to add some
attenuators, in order to get dB levels below -35dBm, which is,
btw, strong enough to get signal lock.

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