libbladeRF: Removed duplicate code from sync_deinit()

The switch() statement here was a vestige from when there were
different condition variables (buf_produced/buf_consumed) that would
need to be passed to sync_worker_deinit().
libbladeRF: Set stream error code on unexpected libusb transfer error
libbladeRF: Propagate stream failure back to sync_rx/tx caller

The error codes from stream failures now work their way back to the
bladerf_sync_rx/tx caller.  Previously, such a failure would either
cause these calls to timeout (when a nonzero timeout_ms parameter was
provided), or (incorrectly) lock up waiting on a buffer while the stream
worker was idle.
libbladeRF: Reduced Error/Warning log level to Debug for lower-level errors

These messages don't mean much to folks not intimately involved with the
underlying libbladeRF code base, and the return codes already indicate
that a failure occurred.

Therefore, the Debug level is more appropriate here.