wifi: mt76: connac: add new definition of tx descriptor

Add MT_TXD6_MSDU_CNT_V2 bitfield, which is used by mt7992 chipsets.
This is a preliminary patch for mt7992 chipsets support.

Signed-off-by: Benjamin Lin <benjamin-jw.lin@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>