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.