ath5k: Implement antenna control

* Add code to support the various antenna scenarios supported by hw

 * For now hardcode the default scenario (single or dual omnis with
 tx/rx diversity working and tx antenna handled by session -hw keeps
 track on which antenna it got ack from each ap/station and maps each
 ap/station to one of the antennas-).

 Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
 Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>