[media] cxd2841er: adjust the dB scale for DVB-C

[media] cxd2841er: adjust the dB scale for DVB-C

Instead of using a relative frequency range, calibrate it to
show the results in dB. The callibration was done getting
samples with a signal generated from -50dBm to -12dBm,
incremented in steps of 0.5 dB, using 3 frequencies:
175 MHz, 410 MHz and 800 MHz. The modulated signal was
using QAM64, and it was used a linear interpolation of all
the results.

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