irqchip/bcm2836: Fix IPI acknowledgement after conversion to handle_percpu_devid_irq

It appears that despite its name, the bcm2836_arm_irqchip_ipi_eoi()
callback is an acknowledgement, and not an EOI. This means that
we lose IPIs that are made pending between the handling of the
IPI and the write to LOCAL_MAILBOX0_CLR0. With the right timing,
things fail nicely.

This used to work with handle_percpu_devid_fasteoi_ipi(), which
started by eoi-ing the interrupt. With the standard fasteoi flow,
this doesn't work anymore.

So let's use this callback for what it is, an ack. Your favourite
RPi-2/3 is back up and running.

Fixes: ffdad793

 ("irqchip/bcm2836: Make IPIs use handle_percpu_devid_irq()")
Cc: Valentin Schneider <valentin.schneider@arm.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/c9fb4ab3-a5cb-648c-6de3-c6a871e60870@roeck-us.net