My previous commits didn't handle the specific case of hackrf_close()
being called without the transfers being active.
In this instance the transfers haven't been setup, so calling
cancel_transfers() returned an error.
Instead:
* refactor out the tx/rx stop command from canceling transfers
* send the tx/rx stop command without canceling transfers, since ..
* ... we can then destroy the transfer thread.
I may also need to put an explicit cancel_transfers() before the
call to send the tx/rx stop commands; I'll look at doing that
in a subsequent commit.
61a06b904d