mt76: mt7921: enlarge maximum VHT MPDU length to 11454

Enlarge maximum MPDU length to 11454 that both mt7921/mt7922 can support.
After this fixing, we can get better performance.

Fixes: 5c14a5f9 ("mt76: mt7921: introduce mt7921e support")
Tested-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>