net/mlx5e: TC, Fix use after free in mlx5e_clone_flow_attr_for_post_act()

This returns freed memory leading to a use after free.  It's supposed to
return NULL.

Fixes: 8300f225 ("net/mlx5e: Create new flow attr for multi table actions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>