RDMA/mlx5: Use representor E-Switch when getting netdev and metadata

Now that a pointer to the managing E-Switch is stored in the representor
use it.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Acked-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>