irqchip/loongson-pch-pic: Check return value of irq_domain_translate_twocell()

Check the return value of irq_domain_translate_twocell() due to
it may returns -EINVAL if failed and use variable fwspec for it,
and then use a new variable parent_fwspec which is proper for
irq_domain_alloc_irqs_parent().

Fixes: ef8c01eb ("irqchip: Add Loongson PCH PIC controller")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1594087972-21715-6-git-send-email-yangtiezhu@loongson.cn