RDMA/mlx5: Refactor netdev affinity code

The design of representors is such that once an IB representor is created,
the netdev of representor already exists, we can use that fact to simplify
the netdev affinity code.

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