ath9k: fix getting tx duration for dynack

On AR9003, tx control and tx status are in separate descriptor rings.
Tx duration is extracted from the tx control descriptor data, which
ar9003_hw_proc_txdesc cannot access.

Fix getting the duration by adding a separate callback for it.

Acked-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
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 7193a00..151ae49 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -683,6 +683,8 @@
 	if (bf_is_ampdu_not_probing(bf))
 		txq->axq_ampdu_depth--;
 
+	ts->duration = ath9k_hw_get_duration(sc->sc_ah, bf->bf_desc,
+					     ts->ts_rateindex);
 	if (!bf_isampdu(bf)) {
 		if (!flush) {
 			info = IEEE80211_SKB_CB(bf->bf_mpdu);