mt76: mt7921: fix max aggregation subframes setting

The hardware can only handle 64 subframes in rx direction and 128 for tx.
Improves throughput with APs that can handle more than that

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210507100211.15709-2-nbd@nbd.name