irqchip/loongson-pch-pic: Fix the misused irq flow handler

Loongson PCH PIC is a standard level triggered PIC, and it need to clear
interrupt during unmask.

Fixes: ef8c01eb ("irqchip: Add Loongson PCH PIC controller")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://lore.kernel.org/r/1596099090-23516-6-git-send-email-chenhc@lemote.com