mt76: mt7921: fix aggregation subframes setting to HE max

mt7921/mt7922 support HE max aggregation subframes 256 for both tx/rx.
Get better throughput then before.

Fixes: 94bb18b0 ("mt76: mt7921: fix max aggregation subframes setting")
Tested-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Reviewed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>