libbladeRF: introduce optimal gain functions
libbladeRF: add auto filtbank selection API

This commit introduces 1dB and 3dB rolloff point based filterbank selection.
bladerf_set_frequency() will set the appropriate filter based on frequency.
libbladeRF: make bladerf_get_frequency() XB-200 compatible

bladerf_get_frequency() needs to be aware if the XB-200 is connected, and to
compensate for the frequency offset if it is.
libbladeRF: fix bladerf_xb200_get_path()

bladerf_xb200_get_path() was returning the opposite path of the actual signal
through the XB-200.
bladeRF-cli: allow cli to go lower than 300MHz

bladeRF-cli now relies on bladerf_set_frequency() to determine if a frequency
selection is invalid.