ath9k: enable CSA functionality in ath9k

CSA is only enabled for one interface, but the same limitation applies
for mac80211 too. It checks whether the beacon has been sent (different
approaches for non-EDMA-enabled and EDMA-enabled devices), and completes
the channel switch after that.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>