SM6FBQ / HackRF clone - History

Don't repeat file that can't advance its position.

This avoids a confusing condition when hackrf_transfer is reading from
stdin with -R in which it does not repeat previous data but repeatedly
restarts reading new data after the user types Ctrl-D to indicate EOF.

The -R option should repeat previous data or should do nothing if the
input file cannot be rewound.

Revert rename of byte_count now that we only have one count.

Fold transfer_finished function into libusb transfer callback.

Expand lock scope in libusb transfer callback.

This is required since cancel_transfers will also cancel the flush.

Fix empty report when TX ends soon after the last report.

Provide a callback for completion of a TX transfer.