soc: mediatek: knows_txdone needs to be set in Mediatek CMDQ helper

Mediatek CMDQ driver have a mechanism to do TXDONE_BY_ACK,
so we should set knows_txdone.

Fixes:576f1b4b ("soc: mediatek: Add Mediatek CMDQ helper")

Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>