net/mlx5: mitigate Tx queue reference counters

net/mlx5: mitigate Tx queue reference counters

The Tx queue structures manage 2 different reference counter per queue:
txq_ctrl reference counter and txq_obj reference counter.

There is no real need to use two different counters, it just complicates
the release functions.
Remove the txq_obj counter and use only the txq_ctrl counter.

Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>