net/mlx5: fix Tx queue completion on stop

net/mlx5: fix Tx queue completion on stop

The Tx queue completion production index was not reset
on Tx queue stop and there were completions remaining
from the previous queue run. This caused the wrong
completion queue operating and overall Tx queue malfunction
on queue restart.

Fixes: 161d103b231c ("net/mlx5: add queue start and stop")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>