gpio: pxa: Fix return value of pxa_gpio_probe()

When call function devm_platform_ioremap_resource(), we should use IS_ERR()
to check the return value and return PTR_ERR() if failed.

Fixes: 542c25b7 ("drivers: gpio: pxa: use devm_platform_ioremap_resource()")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>