pinctrl: digicolor: convert null test to IS_ERR test

Since commit 323de9e ("pinctrl: make pinctrl_register() return proper
error code"), pinctrl_register returns an error code rather than NULL on
failure.  Update a driver that was introduced more recently.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e,e1,e2;
@@

e = pinctrl_register(...)
... when != e = e1
if (
-   e == NULL
+   IS_ERR(e)
   ) {
     ...
     return
-      e2
+      PTR_ERR(e)
     ;
     }
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-digicolor.c

@@ -337,9 +337,9 @@ static int dc_pinctrl_probe(struct platform_device *pdev)
 	pmap->dev = &pdev->dev;
 
 	pmap->pctl = pinctrl_register(pctl_desc, &pdev->dev, pmap);
-	if (!pmap->pctl) {
+	if (IS_ERR(pmap->pctl)) {
 		dev_err(&pdev->dev, "pinctrl driver registration failed\n");
-		return -EINVAL;
+		return PTR_ERR(pmap->pctl);
 	}
 
 	ret = dc_gpiochip_add(pmap, pdev->dev.of_node);