gpio: etraxfs: Fix devm_ioremap_resource return value check

Value returned by devm_ioremap_resource() was checked for non-NULL but
devm_ioremap_resource() returns IOMEM_ERR_PTR, not NULL. In case of
error this could lead to dereference of ERR_PTR.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>