[media] dvb-frontends/cxd2841er: improved snr reporting

On DVB-T/T2 at least, SNR might be reported as >2500dB, which not only is
just wrong but also ridiculous, so fix this by improving the conversion
of the register value.

The INTLOG10X100 function/macro and the way the values are converted were
both taken from DD's cxd2843 driver.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Acked-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>