mailbox: mtk-cmdq: Add struct cmdq_pkt in struct cmdq_cb_data

Current client use 'struct cmdq_pkt' as callback data, so
change 'void *data' to 'struct cmdq_pkt *pkt'. Keep data
until client use pkt instead of data.

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>