mailbox: mediatek: cmdq: clear task in channel before shutdown

Do success callback in channel when shutdown. For those task not finish,
callback with error code thus client has chance to cleanup or reset.

Signed-off-by: Dennis YC Hsieh <dennis-yc.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>