net/mlx5: Return mdev from eswitch

Export a function so users can retrieve the mellanox device that manages
the eswitch from the eswitch device.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>