pinctrl: nuvoton: Fix return value check in wpcm450_gpio_register()

pinctrl: nuvoton: Fix return value check in wpcm450_gpio_register()

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

Fixes: a1d1e0e3d80a ("pinctrl: nuvoton: Add driver for WPCM450")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20220317065851.495394-1-zhangjialin11@huawei.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>