irqchip/mbigen: Unify the error handling in mbigen_of_create_domain()

Dan Carpenter reported that commit fea087fc "irqchip/mbigen: move
to use bus_get_dev_root()" leads to the following Smatch static checker
warning:

	drivers/irqchip/irq-mbigen.c:258 mbigen_of_create_domain()
	error: potentially dereferencing uninitialized 'child'.

It should not cause a problem on real hardware, but better to fix the
warning, let's move the bus_get_dev_root() out of the loop, and unify
the error handling to silence it.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230505090654.12793-1-wangkefeng.wang@huawei.com