gpio: idt3243x: Fix return value check in idt_gpio_probe()

In case of error, the function devm_platform_ioremap_resource_byname()
returns ERR_PTR() and never returns NULL. The NULL test in the return
value check should be replaced with IS_ERR().

Fixes: 4195926a ("gpio: Add support for IDT 79RC3243x GPIO controller")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>