vdpa/mlx5: fix error code for deleting vlan

Return success if we were able to delete a vlan.  The current code
always returns failure.

Fixes: baf2ad3f ("vdpa/mlx5: Add RX MAC VLAN filter support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Message-Id: <Yp709f1g9NcMBCHg@kili>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eli Cohen <elic@nvidia.com>
Acked-by: Si-Wei Liu <si-wei.liu@oracle.com>