ethdev: fix check of threshold for Tx freeing

ethdev: fix check of threshold for Tx freeing

The parameter tx_free_thresh is not consistent between the drivers:
some use it as rte_eth_tx_burst() requires, some release buffers when
the number of free descriptors drop below this value.
Let's use it as most fast-path code does, which is the latter, and update
comments throughout the code to reflect that.

Signed-off-by: Zoltan Kiss <zoltan.kiss@linaro.org>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>