Cleanup of the ENA driver header file

Remove unused macros and fields - some of them were only initialized,
without further usage.

Implement minor style fixes and add required comments.

On the occasion add missing TX completion counter, which was existing,
but mistakenly remained unused.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12864