LoongArch: Fix cpu name after CPU-hotplug

Don't overwrite the SMBIOS-provided CPU name on coming back from CPU-
hotplug (including S3/S4) if it is already initialized.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>