net/mlx5e: Return a valid errno if can't get lag device index

Change the return value to -ENOENT, to make it more meaningful.

Signed-off-by: Jianbo Liu <jianbol@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>