net/mlx5e: TC, Refactor mlx5e_tc_add_flow_mod_hdr() to get flow attr

net/mlx5e: TC, Refactor mlx5e_tc_add_flow_mod_hdr() to get flow attr

In later commit we are going to instantiate multiple attr instances
for flow instead of single attr.
Make sure mlx5e_tc_add_flow_mod_hdr() use the correct attr and not flow->attr.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>