mailbox: imx: Fix an IS_ERR() vs NULL bug

The devm_kzalloc() function does not return error pointers, it returns
NULL on failure.

Fixes: 97961f78 ("mailbox: imx: support i.MX8ULP S4 MU")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>