mt76: mt76u: extend RX scatter gather number

Set RX scatter gather number to 4 in order to extend the maximum AMSDU
size to 11,454.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>