mt76: avoid rx reorder buffer overflow

Enlarge slot to support 11ax 256 BA (256 MPDUs in an AMPDU)

Signed-off-by: Chih-Min Chen <chih-min.chen@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>