net/ena: support Tx mbuf free on demand

net/ena: support Tx mbuf free on demand

ENA driver did not allow applications to call tx_cleanup. Freeing Tx
mbufs was always done by the driver and it was not possible to manually
request the driver to free mbufs.

Modify ena_tx_cleanup function to accept maximum number of packets to
free and return number of packets that was freed.

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