net/mlx5e: Unregister netdev before detaching it

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>