net/mlx5: E-switch, Use helper function to load unload representor

To register and unregister devlink ports when loading/unload
representors, refactor the code to helper functions.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Vu Pham <vuhuong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>