net/mlx5: Organize device list API in one place

Hide the exposed (external) mlx5_dev_list and mlx5_intf_mutex and expose
an organized modular API to manage and manipulate mlx5 devices list.

Signed-off-by: Mohamad Haj Yahia <mohamad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>