wifi: mt76: mt7915: update mpdu density capability

Set mpdu density to 2 usec for non-mt7915 Wi-Fi 6 generation chipsets
to align to the hardware capability which improves the throughput.

Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>