pinctrl: nuvoton: npcm8xx: Add NULL check in npcm8xx_gpio_fw

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.

Fixes: acf4884a5717 ("pinctrl: nuvoton: add NPCM8XX pinctrl and GPIO driver")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Link: https://lore.kernel.org/20250212100532.4317-1-hanchunchao@inspur.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>