Fix getgain incorrect calculation #17
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Fix for issue xtrx-sdr/images#60.
The Rx Gain is being calculated by adding all gains. Having a look at the implementation: https://github.com/pothosware/SoapySDR/blob/fe8dfd1879a8512aa305045ef1e6657a5a33f3b9/lib/Device.cpp#L313-L314
The issue seems to be this line :
https://github.com/xtrx-sdr/libxtrx/blob/4f1eb7b5f77c90bc8d0db2a17fe338372e3d4277/soapy/SoapyXTRX.cpp#L365
On setting the lna gain to 20 and all other gains to 0, the SoapySDR getgain function will return 30 instead.
The root cause is that the minimum gain from the getrange function is being substracted (here -12). As 18-(-12)=30, the negative value seems to cause the issue. Thus setting the minimum at https://github.com/xtrx-sdr/libxtrx/blob/4f1eb7b5f77c90bc8d0db2a17fe338372e3d4277/soapy/SoapyXTRX.cpp#L365 to 0 instead of -12 does fix that issue.