mt76: fix possible OOB issue in mt76_calculate_default_rate

Cap max offset value to ARRAY_SIZE(mt76_rates) - 1 in
mt76_calculate_default_rate routine in order to avoid possible Out Of
Bound accesses.

Fixes: 33920b2b ("mt76: add support for setting mcast rate")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mac80211.c

@@ -1494,7 +1494,6 @@ EXPORT_SYMBOL_GPL(mt76_init_queue);
 u16 mt76_calculate_default_rate(struct mt76_phy *phy, int rateidx)
 {
 	int offset = 0;
-	struct ieee80211_rate *rate;
 
 	if (phy->chandef.chan->band != NL80211_BAND_2GHZ)
 		offset = 4;
@@ -1503,9 +1502,11 @@ u16 mt76_calculate_default_rate(struct mt76_phy *phy, int rateidx)
 	if (rateidx < 0)
 		rateidx = 0;
 
-	rate = &mt76_rates[offset + rateidx];
+	rateidx += offset;
+	if (rateidx >= ARRAY_SIZE(mt76_rates))
+		rateidx = offset;
 
-	return rate->hw_value;
+	return mt76_rates[rateidx].hw_value;
 }
 EXPORT_SYMBOL_GPL(mt76_calculate_default_rate);