vfio/spapr_tce: Make symbol 'tce_iommu_driver_ops' static

Fixes the following sparse warning:

drivers/vfio/vfio_iommu_spapr_tce.c:1401:36: warning:
 symbol 'tce_iommu_driver_ops' was not declared. Should it be static?

Fixes: 5ffd229c ("powerpc/vfio: Implement IOMMU driver for VFIO")
Signed-off-by: Wang Hai <wanghai26@huawei.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>