MIPS: KVM: Remove unneeded volatile

The keyword volatile for idx in the TLB functions is unnecessary.

Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>