net/mlx5e: Gather common netdev init/cleanup functionality in one place

Introduce a helper init/cleanup function that initializes mlx5e generic
netdev private structure, and use them from all profiles init/cleanup
callbacks.

This patch will also be helpful to initialize/cleanup netdevs that are
not created by mlx5 driver, e.g: accelerated ipoib child netdevs.

Fixes: 26e59d80 ("net/mlx5e: Implement mlx5e interface attach/detach callbacks")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>