RDMA/mad: Reduce MAD scope to mlx5_ib only

Management Datagram Interface (MAD) is applicable
only when physical port is Infiniband. It makes MAD
command logic to be completely unrelated to eth/core
parts of mlx5.

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