iWarp devices do not support the creation of address handles
so return AH_ATTR_TYPE_UNDEFINED for all iWarp devices.

While we are here reduce the size of port_num to u8 and add
a comment.

Fixes: 44c58487 ("IB/core: Define 'ib' and 'roce' rdma_ah_attr types")
Reported-by: NParav Pandit <parav@mellanox.com>
CC: Sean Hefty <sean.hefty@intel.com>
Reviewed-by: NIra Weiny <ira.weiny@intel.com>
Reviewed-by: NShiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: NDon Hiatt <don.hiatt@intel.com>
Signed-off-by: NDennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>