mac80211: remove hw.conf.channel usage where possible

Removes hw.conf.channel usage from the following functions:
 * ieee80211_mandatory_rates
 * ieee80211_sta_get_rates
 * ieee80211_frame_duration
 * ieee80211_rts_duration
 * ieee80211_ctstoself_duration

This is in preparation for multi-channel operation.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath5k/qcu.c b/drivers/net/wireless/ath/ath5k/qcu.c
index a6de200..65fe929 100644
--- a/drivers/net/wireless/ath/ath5k/qcu.c
+++ b/drivers/net/wireless/ath/ath5k/qcu.c
@@ -565,6 +565,7 @@
 int ath5k_hw_set_ifs_intervals(struct ath5k_hw *ah, unsigned int slot_time)
 {
 	struct ieee80211_channel *channel = ah->ah_current_channel;
+	enum ieee80211_band band;
 	struct ieee80211_rate *rate;
 	u32 ack_tx_time, eifs, eifs_clock, sifs, sifs_clock;
 	u32 slot_time_clock = ath5k_hw_htoclock(ah, slot_time);
@@ -600,11 +601,12 @@
 	 * Also we have different lowest rate for 802.11a
 	 */
 	if (channel->band == IEEE80211_BAND_5GHZ)
-		rate = &ah->sbands[IEEE80211_BAND_5GHZ].bitrates[0];
+		band = IEEE80211_BAND_5GHZ;
 	else
-		rate = &ah->sbands[IEEE80211_BAND_2GHZ].bitrates[0];
+		band = IEEE80211_BAND_2GHZ;
 
-	ack_tx_time = ath5k_hw_get_frame_duration(ah, 10, rate, false);
+	rate = &ah->sbands[band].bitrates[0];
+	ack_tx_time = ath5k_hw_get_frame_duration(ah, band, 10, rate, false);
 
 	/* ack_tx_time includes an SIFS already */
 	eifs = ack_tx_time + sifs + 2 * slot_time;