net/mlx5: do not select legacy MPW implicitly

The Legacy MPW (multi-packet write) should not be engaged implicitly.
We should exclude this function from a Tx burst routine selection
process unless it is requested specifically by setting the txq_mpw_en
devarg.  Exclude this function from the selection process the same way
it is done for the Enhanced MPW in the mlx5_select_tx_function()
routine.

Fixes: eb8121ab9dac ("net/mlx5: introduce Tx burst routine template")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>