Make getFrequencyRange aware of directSamplingMode

[ryansuchocki]

I've extended the implementation of getFrequencyRange() to be aware of direct sampling mode.

This uses librtlsdr's rtlsdr_get_xtal_freq(). The librtlsdr header indicates that, in direct sampling mode, the tune-able range is from zero to the xtal frequency.

This seems to be necessary in order to get GQRX to let me tune below 28MHz when using SoapyRTLSDR over SoapyRemote.

[zuckschwerdt]

Thanks. LGTM!
Haven't thought about it before, but I guess we should not expect a fixed return from getFrequencyRange().
For reference, BladeRF does something like this too:
https://github.com/pothosware/SoapyBladeRF/blob/85f6dc554ed4c618304d99395b19c4e1523675b0/bladeRF_Settings.cpp#L532-L545