net/mlx5: fix Tx stats error counter definition

net/mlx5: fix Tx stats error counter definition

The current Tx error counter counts, according to its description,
the total number of packets not sent when TX ring full. It is reported
to application as part of oerrors field.

The drop due to full ring is not the statistic that should be set on
oerrors field. Such number can be counted by the application using the
return value of the Tx burst function.
The number that should be set there is the number of packets the device
could not transmit in any way, even when it has resources.

Therefore, replace this counter to count the total number of failed
transmitted packets.

Fixes: 87011737b715 ("mlx5: add software counters")
Cc: stable@dpdk.org
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>