libbladeRF: Fixed LPF mode readback

This became inconsistent when the LPF disabled mode was updated
to both disable bypass and power down the LPF in a5402b78.
libbladeRF: Added simple set & readback tests for control functions

This program performs some simple operations with the various libbladeRF
"control" functions. It is mainly intended to only check for the
presence of grossly erroneous code in these functions, that would cause
crashes or bizarre results.
libbladeRF: Account for FPGA offset of gain correction value