mt76: mt7615: fix fixed-rate tx status reporting

[ Upstream commit ec8f1a90d006f7cedcf86ef19fd034a406a213d6 ]

Rely on the txs fixed-rate bit instead of info->control.rates

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>