pinctrl: sh-pfc: Let gpio_chip.to_irq() return zero on error

Currrently the gpio_chip.to_irq() callback returns -ENOSYS on error,
which causes bad interactions with the serial_mctrl_gpio helpers.

mctrl_gpio_init() returns -ENOSYS if GPIOLIB is not enabled, which is
intended to be ignored by its callers. However, ignoring -ENOSYS when it
was caused by a gpiod_to_irq() failure will lead to a crash later:

    Unable to handle kernel paging request at virtual address ffffffde
    ...
    PC is at mctrl_gpio_set+0x14/0x78

Fix this by returning zero instead, like gpiochip_to_irq() does.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>