lms: Allow values diff than 34dB to be set by setRxGain()
lms: Allow values diff than 34dB to be set by setRxGain()

Until now, setRxGain in LMSDevice did not take into account the setter
parameter and was always using hardcoded 34dB, which was experimentally
found to be a good default value.

Let's force that value during initialization, but still allow the upper
layers (controlled by BTS) to set different values. osmo-bts only sends
a SETRXGAIN command (which calls setRxGain in osmo-trx) if a value is
explicitly set in its VTY config, so we are on the safe side if the user
doesn't explicitly configure a desired dB.

Change-Id: I5684e675281a3f581855dbb56d199a6fe238a712

osmocom/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx