gpio: iop: fix devm_ioremap_resource() return value checking

devm_ioremap_resource() returns a pointer to the remapped memory or
an ERR_PTR() encoded error code on failure.  Fix the check inside
iop3xx_gpio_probe() accordingly.

Cc: Lennert Buytenhek <kernel@wantstofly.org>
Cc: Mikael Pettersson <mikpe@it.uu.se>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>