IB/mlx5: Fix missing congestion control debugfs on rep rdma device

Cited commit missed to include low level congestion control related
debugfs stage initialization.  This resulted in missing debugfs entries
for cc_params of a RDMA device.

Add them back.

Fixes: b5ca15ad ("IB/mlx5: Add proper representors support")
Link: https://lore.kernel.org/r/20200227125407.99803-1-leon@kernel.org


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