KVM: MIPS: Make kvm_mips_callbacks const

Make kvm_mips_callbacks fully const as it's now hardcoded to point at
kvm_vz_callbacks, the only remaining the set of callbacks.

Link: https://lore.kernel.org/all/beb697c2-dfad-780e-4638-76b229f28731@linaro.org


Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230224192832.1286267-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>