irqchip/loongson-pch-pic: Fix initialization of HT vector register

In an ACPI-based dual-bridge system, IRQ of each bridge's
PCH PIC sent to CPU is always a zero-based number, which
means that the IRQ on PCH PIC of each bridge is mapped into
vector range from 0 to 63 of upstream irqchip(e.g. EIOINTC).

      EIOINTC N: [0 ... 63 | 64 ... 255]
                  --------   ----------
                      ^          ^
                      |          |
                  PCH PIC N      |
                             PCH MSI N

For example, the IRQ vector number of sata controller on
PCH PIC of each bridge is 16, which is sent to upstream
irqchip of EIOINTC when an interrupt occurs, which will set
bit 16 of EIOINTC. Since hwirq of 16 on EIOINTC has been
mapped to a irq_desc for sata controller during hierarchy
irq allocation, the related mapped IRQ will be found through
irq_resolve_mapping() in the IRQ domain of EIOINTC.

So, the IRQ number set in HT vector register should be fixed
to be a zero-based number.

Cc: stable@vger.kernel.org
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Co-developed-by: liuyun <liuyun@loongson.cn>
Signed-off-by: liuyun <liuyun@loongson.cn>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230614115936.5950-2-lvjianmin@loongson.cn