pinctrl: digicolor: convert null test to IS_ERR test

Since commit 323de9ef ("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>