net/ena: support Tx/Rx free thresholds

net/ena: support Tx/Rx free thresholds

The caller can pass Tx or Rx free threshold value to the configuration
structure for each ring. It determines when the Tx/Rx function should
start cleaning up/refilling the descriptors. ENA was ignoring this value
and doing it's own calculations.

Now the user can configure ENA's behavior using this parameter and if
this variable won't be set, the ENA will continue with the old behavior
and will use it's own threshold value.

The default value is not provided by the ENA in the ena_infos_get(), as
it's being determined dynamically, depending on the requested ring size.

Note that NULL check for Tx conf was removed from the function
ena_tx_queue_setup(), as at this place the configuration will be
either provided by the user or the default config will be used and it's
handled by the upper (rte_ethdev) layer.

Tx threshold shouldn't be used for the Tx cleanup budget as it can be
inadequate to the used burst. Now the PMD tries to release mbufs for the
ring until it will be depleted.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Shai Brandes <shaibran@amazon.com>