irqchip/atmel-aic: Fix irq_retrigger callback return value

The irq_retrigger callback is supposed to return 0 when retrigger
has failed, and a non-zero value otherwise. Tell the core code
that the driver has succedded in using the HW to retrigger the
interrupt.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200310184921.23552-2-maz@kernel.org