mt76: mt7603: fix invalid fallback rates

Only decrement the rate index on duplicate rates if it is not already 0
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7603/mac.c

@@ -639,6 +639,9 @@ void mt7603_wtbl_set_rates(struct mt7603_dev *dev, struct mt7603_sta *sta,
 			    IEEE80211_TX_RC_40_MHZ_WIDTH)
 				continue;
 
+			if (!rates[i].idx)
+				continue;
+
 			rates[i].idx--;
 		}