libbladeRF: Added readback of FW loopback, disable it for LMS loopback

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 and report the FW mode.

bladerf_set_loopback() has also been updated to ensure that the
FW loopback is disabled when switching to an LMS loopback mode.
bladeRF-cli: Added support for enabling/disabling FW loopback