net/mlx5: remove flags setting from flow preparation

net/mlx5: remove flags setting from flow preparation

Even though flow_drv_prepare() takes item_flags and action_flags to be
filled in, those are not used and will be overwritten by parsing of
flow_drv_translate(). There's no reason to keep the flags and fill it.
Appropriate notes are added to the documentation of flow_drv_prepare() and
flow_drv_translate().

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>