soc: mediatek: cmdq: return send msg error code

Return error code to client if send message fail,
so that client has chance to error handling.

Fixes: 576f1b4b

 ("soc: mediatek: Add Mediatek CMDQ helper")
Signed-off-by: Dennis YC Hsieh <dennis-yc.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://lore.kernel.org/r/1583664775-19382-6-git-send-email-dennis-yc.hsieh@mediatek.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>