irqchip/apple-aic: Fix cpumask allocation for FIQs

An emparassing typo: allocating a pointer instead of the object
pointed to. No harm done, as the pointer is large enough for
what we are using the object for, but still...

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220310050238.4478-1-guozhengkui@vivo.com