KVM: SVM: Rename vmplX_ssp -> plX_ssp

The SSP fields in the SEV-ES save area were mistakenly named vmplX_ssp
instead of plX_ssp. Rename these to the correct names as defined in the
APM.

Fixes: 6d3b3d34 ("KVM: SVM: Update the SEV-ES save area mapping")
Signed-off-by: John Allen <john.allen@amd.com>
Link: https://lore.kernel.org/r/20240227200356.35114-1-john.allen@amd.com


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