net/mlx5: fix legacy multi-packet Tx descriptors

net/mlx5: fix legacy multi-packet Tx descriptors

ConnectX-4LX supports multiple packets within the single Tx
descriptor. This feature is named as "Legacy Multi-Packet Write"
and imposes a lot of limitations:

  - no ACLs, it means no NIC Tx Flows are supported and Tx metadata
    become meaningless
  - the required minimal inline data must be zero
  - no SR-IOV, it means no support in E-Switch configurations,
  - no priority and dscp forcing
  - no VLAN insertion
  - no TSO
  - all packets within MPW session must have the same size

This legacy MPW feature is mainly intended for test purposes.
To explicitly engage the feature on ConnectX-4LX the devargs
should be specified:

  - txq_mpw_en=1

This feature was dropped in 19.08, this patch reverts it back.

Fixes: 18a1c20044c0 ("net/mlx5: implement Tx burst template")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>