mailbox: xgene-slimpro: Fix wrong test for devm_kzalloc

BugLink: https://launchpad.net/bugs/1625232

devm_kzalloc() returns NULL on failure.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
(cherry picked from commit a61b37ea)
Signed-off-by: Craig Magina <craig.magina@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/mailbox/mailbox-xgene-slimpro.c

@@ -189,8 +189,8 @@ static int slimpro_mbox_probe(struct platform_device *pdev)
 	int i;
 
 	ctx = devm_kzalloc(&pdev->dev, sizeof(struct slimpro_mbox), GFP_KERNEL);
-	if (IS_ERR(ctx))
-		return PTR_ERR(ctx);
+	if (!ctx)
+		return -ENOMEM;
 
 	platform_set_drvdata(pdev, ctx);