gpio: x-gene: fix devm_ioremap_resource() check

devm_ioremap_resource() returns IOMEM_ERR_PTR() and it never
returns NULL, fix the check to prevent access to invalid
virtual address.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>