The value returned by make_mmio_spte() is a SPTE, it is not a mask.
Name it accordingly.

No functional change intended.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-10-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>