RDMA/mlx5: Don't create IB representors when in multiport RoCE mode

Switchdev mode and mutiport RoCE mode aren't compatible at this point.
Don't create IB reps when a user switches to switchdev mode and the driver
operates in that mode.

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