gpio: ws16c48: Make irq_chip immutable

Kernel warns about mutable irq_chips:

    "not an immutable chip, please consider fixing!"

Make the struct irq_chip const, flag it as IRQCHIP_IMMUTABLE, add the
new helper functions, and call the appropriate gpiolib functions.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>