gpio: syscon: fix a possible NULL dereference

of_match_device could return NULL, and so cause a NULL pointer
dereference later at line 199:
priv->flags = of_id->data;

Reported-by: coverity (CID 1324140)
Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-syscon.c

@@ -187,11 +187,15 @@ MODULE_DEVICE_TABLE(of, syscon_gpio_ids);
 static int syscon_gpio_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
-	const struct of_device_id *of_id = of_match_device(syscon_gpio_ids, dev);
+	const struct of_device_id *of_id;
 	struct syscon_gpio_priv *priv;
 	struct device_node *np = dev->of_node;
 	int ret;
 
+	of_id = of_match_device(syscon_gpio_ids, dev);
+	if (!of_id)
+		return -ENODEV;
+
 	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return -ENOMEM;