ath5k: fix short preamble rate duration value

Subtract the difference in preamble duration (in usec) from the value
returned by ieee80211_generic_frame_duration.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath5k/ath5k.h b/drivers/net/wireless/ath/ath5k/ath5k.h
index 8a06dbd..996d8af 100644
--- a/drivers/net/wireless/ath/ath5k/ath5k.h
+++ b/drivers/net/wireless/ath/ath5k/ath5k.h
@@ -1233,7 +1233,7 @@
 /* Protocol Control Unit Functions */
 /* Helpers */
 int ath5k_hw_get_frame_duration(struct ath5k_hw *ah,
-		int len, struct ieee80211_rate *rate);
+		int len, struct ieee80211_rate *rate, bool shortpre);
 unsigned int ath5k_hw_get_default_slottime(struct ath5k_hw *ah);
 unsigned int ath5k_hw_get_default_sifs(struct ath5k_hw *ah);
 extern int ath5k_hw_set_opmode(struct ath5k_hw *ah, enum nl80211_iftype opmode);