pinctrl: nuvoton: npcm8xx: Add NULL check in npcm8xx_gpio_fw

In the Linux kernel, the following vulnerability has been resolved: pinctrl: nuvoton: npcm8xx: Add NULL check in npcm8xx_gpio_fw devm_kasprintf() calls can return null pointers on failure. But the return values were not checked in npcm8xx_gpio_fw(). Add NULL check in npcm8xx_gpio_fw(), to handle kernel NULL pointer dereference error.