mfd: pm8921: Use IRQCHIP_SKIP_SET_WAKE

We don't need to implement a dummy irq_set_wake op if we just set
IRQCHIP_SKIP_SET_WAKE.

Suggested-by: Josh Cartwright <joshc@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>