Detaching the netdev before unregistering it cause some netdev cleanup
ndos to fail because they check presence of the netdev, so we need to
unregister the netdev first.

Fixes: 26e59d80 ('net/mlx5e: Implement mlx5e interface attach/detach callbacks')
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>