PM / devfreq: tegra30: Add error message for devm_devfreq_add_device()

It's difficult to notice that driver failed to probe by looking at KMSG
when devm_devfreq_add_device() fails. Add explicit error message for this
case.
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

drivers/devfreq/tegra30-devfreq.c

@@ -922,8 +922,10 @@ static int tegra_devfreq_probe(struct platform_device *pdev)
 
 	devfreq = devm_devfreq_add_device(&pdev->dev, &tegra_devfreq_profile,
 					  "tegra_actmon", NULL);
-	if (IS_ERR(devfreq))
+	if (IS_ERR(devfreq)) {
+		dev_err(&pdev->dev, "Failed to add device: %pe\n", devfreq);
 		return PTR_ERR(devfreq);
+	}
 
 	return 0;
 }