RCG: correct dac duotone amplitude

DAC duotone is a straight copy of the ADC duotone signal into a DAC channel. This code takes into account the different bit resolution of the various parts so that DAC duotone amplitude is the same for the various combinations of ADC and DAC.

requested review from @erik.vonreis

assigned to @ezekiel.dohmen

merged

mentioned in commit 96871bf6