irqchip/gic-v2m: Fix of_node refcount on error

On the error path, the v2m drivers drops the refcount on the parent
node instead of doing it on the node that generated the error.
Humph...

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>