irqchip/loongson-eiointc: Fix returned value on parsing MADT

In pch_pic_parse_madt(), a NULL parent pointer will be
returned from acpi_get_vec_parent() for second pch-pic domain
related to second bridge while calling eiointc_acpi_init() at
first time, where the parent of it has not been initialized
yet, and will be initialized during second time calling
eiointc_acpi_init(). So, it's reasonable to return zero so
that failure of acpi_table_parse_madt() will be avoided, or else
acpi_cascade_irqdomain_init() will return and initialization of
followed pch_msi domain will be skipped.

Although it does not matter when pch_msi_parse_madt() returns
-EINVAL if no invalid parent is found, it's also reasonable to
return zero for that.

Cc: stable@vger.kernel.org
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230407083453.6305-2-lvjianmin@loongson.cn