[media] drxk: use a better calculus for RF strength

The AZ6007 driver released by Terratec has a better way to
estimate the signal strength, at CtrlSigStrength(). Port it
to the driver.
It should be noticed that there are two parameters there that
are tuner-specific.

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