iwlwifi: comment that setting driver_data overrides info->control

Using the driver_data area in ieee80211_tx_info which
resides in the CB overrides the info->control field.
Add a comment to prevent mistakes.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/iwlwifi/dvm/tx.c

@@ -403,6 +403,7 @@ int iwlagn_tx_skb(struct iwl_priv *priv, struct sk_buff *skb)
 
 	info->driver_data[0] = ctx;
 	info->driver_data[1] = dev_cmd;
+	/* From now on, we cannot access info->control */
 
 	spin_lock(&priv->sta_lock);