irqchip/armada-370-xp: Fix ack/eoi breakage

When converting the driver to using handle_percpu_devid_irq,
we forgot to repaint the irq_eoi() callback into irq_ack(),
as handle_percpu_devid_fasteoi_ipi() was actually using EOI
really early in the handling. Yes this was a stupid idea.

Fix this by using the HW ack method as irq_ack().

Fixes: e52e73b7 ("irqchip/armada-370-xp: Make IPIs use handle_percpu_devid_irq()")
Reported-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Tested-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/87tuiexq5f.fsf@pengutronix.de