irqchip/loongson-pch-pic: Fix translate callback for DT path

In DT path of translate callback, if fwspec->param_count==1
and of_node is non-null, fwspec->param[1] will be accessed,
which is introduced from previous commit bcdd75c5
(irqchip/loongson-pch-pic: Add ACPI init support).

Before the patch, for non-null of_node, translate callback
(use irq_domain_translate_twocell()) will return -EINVAL if
fwspec->param_count < 2, so the check in the patch is added.

Fixes: bcdd75c5 ("irqchip/loongson-pch-pic: Add ACPI init support")
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221022075955.11726-3-lvjianmin@loongson.cn