mailbox/xgene-slimpro: Checking for IS_ERR instead of NULL

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

devm_ioremap() returns NULL, it never returns an ERR_PTR.

Fixes: f700e84f ('mailbox: Add support for APM X-Gene platform mailbox driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
(cherry picked from commit 14d653af)
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

@@ -196,8 +196,8 @@ static int slimpro_mbox_probe(struct platform_device *pdev)
 
 	regs = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	mb_base = devm_ioremap(&pdev->dev, regs->start, resource_size(regs));
-	if (IS_ERR(mb_base))
-		return PTR_ERR(mb_base);
+	if (!mb_base)
+		return -ENOMEM;
 
 	/* Setup mailbox links */
 	for (i = 0; i < MBOX_CNT; i++) {