libbladeRF: Removed 3.72GHz SW limit. Allow tuning up to 3.8GHz.

This was limited due to 3.72GHz being the maximum value show in the
LMS6002D programming guide's FREQSEL table. According to a response
on the limemicro-opensource Google group, we should be able to use
the same FREQSEL value from 3.72GHz - 3.8GHz.