mt76: mt7915: update max_mpdu_size in mt7915_mcu_sta_amsdu_tlv()

The maximum max_mpdu_size of mt7915 is 7991, whereas mt7916 can
support 11454.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>