mac80211: fix max HT rate processing on mac80211

The max MCS index is 76, fix the higher check to allow through
frames received at MCS 76. This is a non-issue for current drivers
as MCS 76 is only possible with a device supporting 4 spatial
streams.

While at it change the WARN_ON() on invalid HT rates to a WARN()
to provide more useful information. This will help debug issues
when the driver is passing up a bogus HT rate value.

The rate must map to a valid MCS index which can be any of the
values in the set [0 - 76] (inclusive).
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rx.c

@@ -2441,9 +2441,21 @@ void ieee80211_rx(struct ieee80211_hw *hw, struct sk_buff *skb)
 		goto drop;
 
 	if (status->flag & RX_FLAG_HT) {
-		/* rate_idx is MCS index */
-		if (WARN_ON(status->rate_idx < 0 ||
-			    status->rate_idx >= 76))
+		/*
+		 * rate_idx is MCS index, which can be [0-76] as documented on:
+		 *
+		 * http://wireless.kernel.org/en/developers/Documentation/ieee80211/802.11n
+		 *
+		 * Anything else would be some sort of driver or hardware error.
+		 * The driver should catch hardware errors.
+		 */
+		if (WARN((status->rate_idx < 0 ||
+			 status->rate_idx > 76),
+			 "Rate marked as an HT rate but passed "
+			 "status->rate_idx is not "
+			 "an MCS index [0-76]: %d (0x%02x)\n",
+			 status->rate_idx,
+			 status->rate_idx))
 			goto drop;
 		/* HT rates are not in the table - use the highest legacy rate
 		 * for now since other parts of mac80211 may not yet be fully