mac80211: check if atf has been disabled in __ieee80211_schedule_txq

Check if atf has been disabled in __ieee80211_schedule_txq() in order to
avoid a given sta is always put to the beginning of the active_txqs list
and never moved to the end since deficit is not decremented in
ieee80211_sta_register_airtime()

Fixes: b4809e94 ("mac80211: Add airtime accounting and scheduling to TXQs")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://lore.kernel.org/r/93889406c50f1416214c079ca0b8c9faecc5143e.1608975195.git.lorenzo@kernel.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3809,7 +3809,7 @@ void __ieee80211_schedule_txq(struct ieee80211_hw *hw,
 		 * get immediately moved to the back of the list on the next
 		 * call to ieee80211_next_txq().
 		 */
-		if (txqi->txq.sta &&
+		if (txqi->txq.sta && local->airtime_flags &&
 		    wiphy_ext_feature_isset(local->hw.wiphy,
 					    NL80211_EXT_FEATURE_AIRTIME_FAIRNESS))
 			list_add(&txqi->schedule_order,