Evaluate alternate sample rates to reduce phase noise

[aSmig]

gr-osmosdr hackrf_source_c.cc suggests that using 8 MHz, 10 MHz, 12.5 MHz, 16 MHz, or 20 MHz sample rates will produce less phase noise than fractional rates. We are presently sampling at 1.92 MHz as a common multiple of our receive bitrates to simplify resampling. We should evaluate the performance of sample rates believed to reduce phase noise.