mt76: mt7615: fix tx status rate index calculation

A switch from one rate index to the next only happens when tx count from
the current slot is greater than MT7615_RATE_RETRY, which is 1 has to be
subtracted from count, instead of added to it.
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/mac.c

@@ -1148,7 +1148,7 @@ static bool mt7615_fill_txs(struct mt7615_dev *dev, struct mt7615_sta *sta,
 	if (ampdu || (info->flags & IEEE80211_TX_CTL_AMPDU))
 		info->flags |= IEEE80211_TX_STAT_AMPDU | IEEE80211_TX_CTL_AMPDU;
 
-	first_idx = max_t(int, 0, last_idx - (count + 1) / MT7615_RATE_RETRY);
+	first_idx = max_t(int, 0, last_idx - (count - 1) / MT7615_RATE_RETRY);
 
 	if (fixed_rate && !probe) {
 		info->status.rates[0].count = count;