mailbox: mtk-cmdq: remove cl in struct cmdq_pkt

Every client driver has the struct cmdq_client information, so
it's not necessary to store it in struct cmdq_pkt. cl is used to
store struct cmdq_client information and now no client driver use
it, so remove it.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>