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.