mt76: fix possible out-of-bound access in mt7615_fill_txs/mt7603_fill_txs

Fix possible out-of-bound access of status rates array in
mt7615_fill_txs/mt7603_fill_txs routines

Fixes: c5211e99 ("mt76: mt7603: rework and fix tx status reporting")
Fixes: 4af81f02

 ("mt76: mt7615: sync with mt7603 rate control changes")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>