RDMA/mlx5: Move default representors SQ steering to rule to modify QP

Currently the steering for SQs created on representors is done on
creation, once we move to representors as ports of an IB device we need
the port argument which is given only at the modify QP stage, adjust the
code appropriately.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>