net/mlx5: Make load_one() and unload_one() symmetric

Currently mlx5_load_one() perform device registration using
mlx5_register_device(). But mlx5_unload_one() doesn't unregister.

Make them symmetric by doing device unregistration in
mlx5_unload_one().

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>