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: 5ffd229 ("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>

drivers/vfio/vfio_iommu_spapr_tce.c

@@ -1398,7 +1398,7 @@ static void tce_iommu_detach_group(void *iommu_data,
 	mutex_unlock(&container->lock);
 }
 
-const struct vfio_iommu_driver_ops tce_iommu_driver_ops = {
+static const struct vfio_iommu_driver_ops tce_iommu_driver_ops = {
 	.name		= "iommu-vfio-powerpc",
 	.owner		= THIS_MODULE,
 	.open		= tce_iommu_open,