OMAPDSS: HDMI: fix devm_ioremap_resource error checks

devm_ioremap_resource returns ERR_PTR on error, not NULL. Fix the
error checks in the driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/drivers/video/fbdev/omap2/dss/hdmi_wp.c b/drivers/video/fbdev/omap2/dss/hdmi_wp.c
index 706ea90..496327e 100644
--- a/drivers/video/fbdev/omap2/dss/hdmi_wp.c
+++ b/drivers/video/fbdev/omap2/dss/hdmi_wp.c
@@ -249,9 +249,9 @@
 	}
 
 	wp->base = devm_ioremap_resource(&pdev->dev, res);
-	if (!wp->base) {
+	if (IS_ERR(wp->base)) {
 		DSSERR("can't ioremap HDMI WP\n");
-		return -ENOMEM;
+		return PTR_ERR(wp->base);
 	}
 
 	return 0;