rtc: snvs: return error in case enable_irq_wake fails

If enable_irq_wake fails, we should return that error code so that
entering suspend fails. Otherwise we will get a WARNING along with
the hint of a unbalanced wake disable:
Unbalanced IRQ 37 wake disable

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>