gpio: pcf857x: Simpify wake-up handling

Unlike gpio_rcar_irq_set_wake(), which this solution was based on,
pcf857x_irq_set_wake() does not need to do anything else than calling
irq_set_irq_wake() for the upstream interrupt controller.  Hence there
is no reason for making this call conditional, and no longer a need for
keeping a copy of the interrupt number that also serves as a flag.

Just propagate irq_set_irq_wake() to the upstream interrupt controller,
using the original interrupt number.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>