PCI: Enable is not exposed as a PASID capability

The PASID ECN indicates bit 0 is reserved in the capability register.
Switch pci_enable_pasid() to error if PASID is already enabled and
don't expose enable as a feature in pci_pasid_features().

Reviewed-by: Joerg Roedel <joerg.roedel@amd.com>
Tested-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>