soc: mediatek: cmdq: Remove cmdq_pkt_flush()

rx_callback is a standard mailbox callback mechanism and could
cover the function of proprietary cmdq_task_cb, so it is better
to use the standard one instead of the proprietary one. But
register rx_callback should before mbox_request_channel(),
so remove cmdq_pkt_flush() and let client driver implement
its own synchronous flush.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/20201202235856.7652-1-chunkuang.hu@kernel.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>