ath9k: always use SIFS times from OFDM for 5/10 MHz

5/10 MHz channels should always use SIFS times as defined in IEEE
802.11-2012 18.4.4 (OFDM PHY characteristics). This makes it compatible
to ath5k, which does the same.
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/hw.c

@@ -1069,7 +1069,7 @@ void ath9k_hw_init_global_settings(struct ath_hw *ah)
 		if (IS_CHAN_A_FAST_CLOCK(ah, chan))
 		    tx_lat += 11;
 
-		sifstime *= 2;
+		sifstime = 32;
 		ack_offset = 16;
 		slottime = 13;
 	} else if (IS_CHAN_QUARTER_RATE(chan)) {
@@ -1079,7 +1079,7 @@ void ath9k_hw_init_global_settings(struct ath_hw *ah)
 		if (IS_CHAN_A_FAST_CLOCK(ah, chan))
 		    tx_lat += 22;
 
-		sifstime *= 4;
+		sifstime = 64;
 		ack_offset = 32;
 		slottime = 21;
 	} else {
@@ -1116,7 +1116,6 @@ void ath9k_hw_init_global_settings(struct ath_hw *ah)
 		ctstimeout += 48 - sifstime - ah->slottime;
 	}
 
-
 	ath9k_hw_set_sifs_time(ah, sifstime);
 	ath9k_hw_setslottime(ah, slottime);
 	ath9k_hw_set_ack_timeout(ah, acktimeout);