mailbox: mtk-cmdq: Use mailbox rx_callback

rx_callback is a standard mailbox callback mechanism and could cover the
function of proprietary cmdq_task_cb, so use the standard one instead of
the proprietary one. But the client driver has already used cmdq_task_cb,
so keep cmdq_task_cb until all client driver use rx_callback instead of
cmdq_task_cb.

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>