host/common: Fixed leak in error path

Addresses CID 1042550
libbladeRF: Specified calling convention for Windows and removed unneeded externs

Misc. reformatting and a few doxygen touchups also included.
libbladeRF: Fixed a bad ifdef

The libusb_get_version() call here is generally helpful to have
in here, as the libusb version is one of the first things to check
when someone reports USB issues.