rt2x00: Fix TX rate short preamble detection

Mac80211 provides 2 structures to handle bitrates, namely
ieee80211_rate and ieee80211_tx_rate. To determine the short preamble
mode for an outgoing frame, the flag IEEE80211_TX_RC_USE_SHORT_PREAMBLE
must be checked on ieee80211_tx_rate and not ieee80211_rate (which rt2x00 did).

This fixes a regression which was triggered in 2.6.29-rcX as reported by Chris Clayton.
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Tested-By: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00queue.c

@@ -154,6 +154,7 @@ static void rt2x00queue_create_tx_descriptor(struct queue_entry *entry,
 	struct rt2x00_dev *rt2x00dev = entry->queue->rt2x00dev;
 	struct ieee80211_tx_info *tx_info = IEEE80211_SKB_CB(entry->skb);
 	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)entry->skb->data;
+	struct ieee80211_tx_rate *txrate = &tx_info->control.rates[0];
 	struct ieee80211_rate *rate =
 	    ieee80211_get_tx_rate(rt2x00dev->hw, tx_info);
 	const struct rt2x00_rate *hwrate;
@@ -313,7 +314,7 @@ static void rt2x00queue_create_tx_descriptor(struct queue_entry *entry,
 		 * When preamble is enabled we should set the
 		 * preamble bit for the signal.
 		 */
-		if (rate->flags & IEEE80211_TX_RC_USE_SHORT_PREAMBLE)
+		if (txrate->flags & IEEE80211_TX_RC_USE_SHORT_PREAMBLE)
 			txdesc->signal |= 0x08;
 	}
 }