LoongArch: KVM: Avoid overflow with array index

The variable index is modified and reused as array index when modify
register EIOINTC_ENABLE. There will be array index overflow problem.

Cc: stable@vger.kernel.org
Fixes: 3956a52b ("LoongArch: KVM: Add EIOINTC read and write functions")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>