net/ena: perform Tx cleanup before sending packets

net/ena: perform Tx cleanup before sending packets

To increase likelihood that current burst will fit in the HW rings,
perform Tx cleanup before pushing packets to the HW. It may increase
latency a bit for sparse bursts, but the Tx flow now should be more
smooth.

It's also common order in the Tx burst function for other PMDs.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Dawid Gorecki <dgr@semihalf.com>
Reviewed-by: Shai Brandes <shaibran@amazon.com>