mailbox: xgene-slimpro: Fix wrong test for devm_kzalloc

devm_kzalloc() returns NULL on failure.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
diff --git a/drivers/mailbox/mailbox-xgene-slimpro.c b/drivers/mailbox/mailbox-xgene-slimpro.c
index bd07f39..dd2afbc 100644
--- a/drivers/mailbox/mailbox-xgene-slimpro.c
+++ b/drivers/mailbox/mailbox-xgene-slimpro.c
@@ -189,8 +189,8 @@
 	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);