hw/intc/loongarch_pch: Convert to little endian with ID register

hw/intc/loongarch_pch: Convert to little endian with ID register

With PCH ID register, it is defined as union type as follows:
union LoongArchPIC_ID {
    struct {
        uint8_t _reserved_0[3];
        uint8_t id;
        uint8_t version;
        uint8_t _reserved_1;
        uint8_t irq_num;
        uint8_t _reserved_2;
    } QEMU_PACKED desc;
    uint64_t data;
}

And with pch driver in virt machine irq_number is parsed with little
endian method:
  vec_count = ((readq(priv->base) >> 48) & 0xff) + 1

So the value of ID register should be converted to little endian.
With this patch, linux kernel passes to run on S390 big endian host
machine with TCG method.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20250604065502.1114098-3-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>