mailbox: mtk-cmdq: Remove cmdq_cb_status

cmdq_cb_status is an error status. Use the standard error number
instead of cmdq_cb_status to prevent status duplication.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Reviewed-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>