wifi: mt76: mt7996: fix definition of tx descriptor

wifi: mt76: mt7996: fix definition of tx descriptor

For mt7992 chipsets, the definition of TXD.DW6.BIT10~15 has different
interpretations on different frame types. Driver only needs to fill
MSDU_CNT for non-management frames.

Fixes: 408566db8cad ("wifi: mt76: connac: add new definition of tx descriptor")
Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Benjamin Lin <benjamin-jw.lin@mediatek.com>
Link: https://patch.msgid.link/20250114101026.3587702-6-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>