Documentation: pinctrl: Fix example code for pinctrl_register

pinctrl_register() returns NULL on error, fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/Documentation/pinctrl.txt b/Documentation/pinctrl.txt
index e7bee9b..c0ffd30 100644
--- a/Documentation/pinctrl.txt
+++ b/Documentation/pinctrl.txt
@@ -81,7 +81,7 @@
 	struct pinctrl_dev *pctl;
 
 	pctl = pinctrl_register(&foo_desc, <PARENT>, NULL);
-	if (IS_ERR(pctl))
+	if (!pctl)
 		pr_err("could not register foo pin driver\n");
 }