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>

drivers/mailbox/imx-mailbox.c

@@ -563,8 +563,8 @@ static int imx_mu_probe(struct platform_device *pdev)
 		size = sizeof(struct imx_sc_rpc_msg_max);
 
 	priv->msg = devm_kzalloc(dev, size, GFP_KERNEL);
-	if (IS_ERR(priv->msg))
-		return PTR_ERR(priv->msg);
+	if (!priv->msg)
+		return -ENOMEM;
 
 	priv->clk = devm_clk_get(dev, NULL);
 	if (IS_ERR(priv->clk)) {