LoongArch: KVM: Disable updating of "num_cpu" and "feature"

Property "num_cpu" and "feature" are read-only once eiointc is created,
which are set with KVM_DEV_LOONGARCH_EXTIOI_GRP_CTRL attr group before
device creation.

Attr group KVM_DEV_LOONGARCH_EXTIOI_GRP_SW_STATUS is to update register
and software state for migration and reset usage, property "num_cpu" and
"feature" can not be update again if it is created already.

Here discard write operation with property "num_cpu" and "feature" in
attr group KVM_DEV_LOONGARCH_EXTIOI_GRP_CTRL.

Cc: stable@vger.kernel.org
Fixes: 1ad7efa5 ("LoongArch: KVM: Add EIOINTC user mode read and write functions")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>