mt76: mt7663s: fix possible quota leak in mt7663s_refill_sched_quota

Look just at reported quota since the hw sporadically reports mcu tx
quota without setting WHIER_TX_DONE_INT_EN bit

Tested-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>