Having a software timestamp (along with existing hardware one) is
useful to trace how the packets flow through the stack.
mlx5e_tx_skb_update_hwts_flags is called from tx paths
to setup HW timestamp; extend it to add software one as well.
Reviewed-by: Jason Xing <kerneljasonxing@xxxxxxxxx>
Signed-off-by: Stanislav Fomichev <stfomichev@xxxxxxxxx>