ena: Swap cleanup order

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 <ofirt@amazon.com>
MFC after: 2 weeks
Sponsored by: Amazon, Inc.
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D58239