mailbox: mtk-cmdq: Do not request irq until we are ready

If the system comes from kexec() the peripheral might trigger an IRQ
befoe we are ready for it. Triggering a crash due to an access to
invalid memory.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>