ath9k: add a workaround for the tx ack time issue triggered by the timing handling cleanups

in 2.4 ghz, calculating the ack timeout the way it is described in 802.11-2007 currently leads to tx timeouts
set it to a minimum of what the initvals set (64 usec) until we know what is causing this

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@19156 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/mac80211/patches/580-ath9k_ack_timeout_workaround.patch

@@ -0,0 +1,14 @@
+--- a/drivers/net/wireless/ath/ath9k/hw.c
++++ b/drivers/net/wireless/ath/ath9k/hw.c
+@@ -1229,6 +1229,11 @@ void ath9k_hw_init_global_settings(struc
+ 	/* As defined by IEEE 802.11-2007 17.3.8.6 */
+ 	slottime = ah->slottime + 3 * ah->coverage_class;
+ 	acktimeout = slottime + sifstime;
++
++	/* Workaround for a hw issue */
++	if (conf->channel && conf->channel->band == IEEE80211_BAND_2GHZ)
++		acktimeout = max(64, acktimeout);
++
+ 	ath9k_hw_setslottime(ah, slottime);
+ 	ath9k_hw_set_ack_timeout(ah, acktimeout);
+ 	ath9k_hw_set_cts_timeout(ah, acktimeout);