LoongArch: KVM: Remove undefined a6 argument comment for kvm_hypercall()

The kvm_hypercall() set for LoongArch is limited to a1-a5. So the
mention of a6 in the comment is undefined that needs to be rectified.

Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Dandan Zhang <zhangdandan@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>