net/ena: refactor Tx

net/ena: refactor Tx

The original Tx function was very long and was containing both cleanup
and the sending sections. Because of that it was having a lot of local
variables, big indentation and was hard to read.

This function was split into 2 sections:
  * Sending - which is responsible for preparing the mbuf, mapping it
    to the device descriptors and finally, sending packet to the HW
  * Cleanup - which is releasing packets sent by the HW. Loop which was
    releasing packets was reworked a bit, to make intention more visible
    and aligned with other parts of the driver.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>