irqchip/irq-mbigen: Add of_node_put() before return

Each iteration of for_each_child_of_node puts the previous node, but
in the case of a return from the middle of the loop, there is no put,
thus causing a memory leak. Add an of_node_put before the return in
three places.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>