ath9k: prepare for multi-interface CSA support

Soon mac80211 will support multi-interface CSA so
using sc->csa_vif is not an option.

Instead just depend on vif->csa_active. Calling
ieee80211_csa_finish() multiple number of times
should not be an issue.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
(cherry picked from commit http://mid.gmane.org/1390228033-19162-2-git-send-email-michal.kazior@tieto.com)