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>

drivers/net/wireless/rtl818x/rtl8187/dev.c

@@ -294,6 +294,7 @@ static void rtl8187_tx(struct ieee80211_hw *dev, struct sk_buff *skb)
 		hdr->retry = cpu_to_le32((info->control.rates[0].count - 1) << 8);
 		hdr->tx_duration =
 			ieee80211_generic_frame_duration(dev, priv->vif,
+							 info->band,
 							 skb->len, txrate);
 		buf = hdr;