irqchip/mbigen: Simplify code logic with for_each_child_of_node_scoped()

for_each_child_of_node_scoped() handles the device_node automaticlly, so
switching over to it removes the device node cleanups and allows to return
directly from the loop.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240808031552.3156-1-zhangzekun11@huawei.com