net/mlx5e: TC, Use common function allocating flow mod hdr or encap mod hdr

Use mlx5e_tc_attach_mod_hdr() when allocating encap mod hdr and
remove mlx5e_tc_add_flow_mod_hdr() which is not being used now.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>