iommu/amd: Fix a stale comment about which legacy mode is user visible

iommu/amd: Fix a stale comment about which legacy mode is user visible

Update a stale comment about which of the legacy modes is visible to the
user, i.e. can be forced via amd_iommu_intr=legacy.

Fixes: b74aa02d7a30 ("iommu/amd: Fix legacy interrupt remapping for x2APIC-enabled system")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Wei Wang <wei.w.wang@hotmail.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>