bladeRF-cli: fix occasional CLI hangups

Occasionally, bladeRF-cli --help or --version will hang and not exit properly.  This seems most common if I don't have my bladeRF plugged in.

I believe it was attempting to do rxtx_shutdown() even if there was no reason to do so (e.g. there was no bladeRF hardware connected).  Use convenience functions to determine if we need to do it.