net/mlx5: Lag, fix a potential Oops with mlx5_lag_create_definer()

There is a minus character missing from ERR_PTR(ENOMEM) so if this
allocation fails it will lead to an Oops in the caller.

Fixes: dc48516e ("net/mlx5: Lag, add support to create definers for LAG")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>