pinctrl: ingenic: checking for NULL instead of IS_ERR()

devm_pinctrl_register() returns error pointers, it doesn't return NULL.

Fixes: b5c23aa4 ("pinctrl: add a pinctrl driver for the Ingenic jz47xx SoCs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>