vfio/spapr_tce: Fix the comment

Grepping for "iommu_ops" finds this spot and gives wrong impression
that iommu_ops is used in here, fix the comment.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Link: https://lore.kernel.org/r/20220714080912.3713509-1-aik@ozlabs.ru


[aw: convert to multi-line comment]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>