wifi: mt76: mt7996: rework mt7996_mac_write_txwi() for MLO support

wifi: mt76: mt7996: rework mt7996_mac_write_txwi() for MLO support

Update mt7996_mac_write_txwi routine and all the called subroutines to
support MLO.
This is a preliminary patch to enable MLO for MT7996 driver

Co-developed-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Co-developed-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250312-b4-mt7996-mlo-p2-v1-4-015b3d6fd928@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>