[media] xc4000: add support for signal strength measures

[media] xc4000: add support for signal strength measures

In xc4000 chipsets real signal and noise level is stored in register
0x0A and 0x0B,so we can use those registers to monitor signal strength.

I tested this patch on 2 different cards Leadtek DVR3200 and DTV2000H
Plus, both with same results, I used special antenna hubs (toner 4x, 6x,
8x and 12x) with mesured signal lost, both registers are in dB value,
first represent signal with limit value -113.5dB (should be -114dB) and
exactly match with test results. Second represents noise level also in
dB and there is no maximum value, but from tests we can drop everything
above 32dB which tuner realy can't use, signal was usable till 20dB
noise level.

In digital mode we can take signal strength but sadly noise level is not
relevant and real value is stored in demodulator for now just zl10353,
also digital mode is just for testing, because it needs changing other
parts of code which reads data only from demodulator.

In analog mode I was able to test only FM radio, signal level is not
important, it says something about cable and hub losts, but nothing
about real quality of reception, so even if we have signal level at
minimum 113dB we can still here radio, because of that it is displaied
only in debug mode, but for real signal level is used noise register
which is again very accurate, radio noise level was betwen 6-20dB for
good signal, 20-25dB for medium signal, and above 25dB signal is
unusable.

For now real benefit of this patch is only for FM radio mode.

Signed-off-by: Miroslav Slugen <thunder.mmm@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>