IB/core: Map iWarp AH type to undefined in rdma_ah_find_type

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: Parav Pandit <parav@mellanox.com>
CC: Sean Hefty <sean.hefty@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>