ath9k: cleanup slot time and ack/cts timeout handling

Previously ath9k left the initialization of slot timing and ACK/CTS
timeout to the mode specific initvals. This does not handle short vs
long slot in 2.4 GHz and uses a rather strange value for the 2.4 GHz
ACK timeout (64 usec).
This patch uses the proper ath9k_hw functions for setting slot time and
timeouts and also implements the switch between short and long slot
time in 2.4 GHz

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/beacon.c b/drivers/net/wireless/ath/ath9k/beacon.c
index 1660ef1..422454f 100644
--- a/drivers/net/wireless/ath/ath9k/beacon.c
+++ b/drivers/net/wireless/ath/ath9k/beacon.c
@@ -480,7 +480,8 @@
 		sc->beacon.updateslot = COMMIT; /* commit next beacon */
 		sc->beacon.slotupdate = slot;
 	} else if (sc->beacon.updateslot == COMMIT && sc->beacon.slotupdate == slot) {
-		ath9k_hw_setslottime(sc->sc_ah, sc->beacon.slottime);
+		ah->slottime = sc->beacon.slottime;
+		ath9k_hw_init_global_settings(ah);
 		sc->beacon.updateslot = OK;
 	}
 	if (bfaddr != 0) {