ena: Swap cleanup order

[akiyano_amazon.com]

Summary

As RX processing is heavier than TX completions processing, swap the
order and process TX completions first, in order to avoid starving the
completions and causing potential missing TX completions.

Submitted by: Ofir Tabachnik <[e-mail]>
MFC after: 2 weeks
Sponsored by: Amazon, Inc.

[cperciva]

I'm not convinced by the rationale here -- it seems to me that the stronger argument is "processing tx completions frees up space on the tx ring, which we might want when we respond to packets which we received". But the change itself seems reasonable.