libbladeRF: Added readback of FW loopback, ensure mutually exclusive LB

Added the ability to read back the state of FW loopback from devices
running FW v1.7.1. While it is possible to enable FW loopback mode in
v1.7.0 firmware, libbladeRF will now enforce the v1.7.1 requirement so
it can correctly read back the FW mode.

bladerf_set_loopback() has also been fixed to ensure that the
FW and LMS loopback functions are enabled in a mutually exclusive
manner.