Override setGain and getGainRange to avoid changing RFGR.

[dlaw]

SDRPlay's "RFGR" is actually a mode selector for the front-end LNA. It does not have units of dB. (In fact, the resulting gain is frequency-dependent.) As a result, the default gain allocation algorithm does not apply and only IFGR should be used for automatic adjustment of the overall system gain.

(See https://github.com/pothosware/SoapySDRPlay2/issues/60 for previous discussion, https://www.sdrplay.com/docs/SDRplay_SDR_API_Specification.pdf section 5.3 for the definition of RFGR.)