mailbox: imx: Modify the incorrect format specifier

Replace %i with %u in snprintf() because it is "unsigned int".

Signed-off-by: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>