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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
index 0a75e2f..a650704 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -2566,7 +2566,7 @@
 			sc->beacon.tx_processed = true;
 			sc->beacon.tx_last = !(ts.ts_status & ATH9K_TXERR_MASK);
 
-			ath9k_csa_is_finished(sc);
+			ath9k_csa_update(sc);
 			continue;
 		}