mailbox: imx-mailbox: fix scu msg header size check

The i.MX8 SCU message header size is the number of "u32" elements,
not "u8", so fix the check.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1461658 ("Memory - corruptions")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>