sPAPR/IOMMU: Fix TCE entry permission

The permission of TCE entry should exclude physical base address.
Otherwise, unmapping TCE entry can be interpreted to mapping TCE
entry wrongly for VFIO devices.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>