Transceiver: use proper factor for amplitude scaling

In Transceiver::addRadioVector() we scale the I/Q samples by scaling
the output voltage of the DAC.   A relative factor/divisor/ration in
the voltage domain cannot be used 1:1 in the power domain.

There exist two similar formulas:

  a) X_dB = 10 * log10(X_lin / X_ref)
  b) Y_db = 20 * log10(Y_lin / Y_ref)

both of them are correct, and according to [1]:

  a) If you convert a quantity X that relates to power or energy,
     => the factor is 10.
  b) If you convert a quantity Y that relates to amplitude,
     => the factor is 20.

Therefore we should be using 20 instead of 10.  This change makes
osmo-trx apply per-lchan attenuation values correctly.  Otherwise
it would double the values indicated in TRXD messages.

[1] https://dspillustrations.com/pages/posts/misc/decibel-conversion-factor-10-or-factor-20.html

Change-Id: I98bc00bd25df4913d45e55eb008d715aca76fc7c
Related: SYS#4918

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