RDMA/siw: Enable siw on tunnel devices

Enable siw to attach to tunnel devices, there is no reason not to, siw
properly generates all packets already.

Link: https://lore.kernel.org/r/20220510143917.23735-1-bmt@zurich.ibm.com
Tested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>