Cleanup of the ENA driver header file

[mk_semihalf.com]

Summary

Remove unused macros and fields - some of them were only initialized,
but had no further uses.

Minor style fixes and added required comments.

Missing TX completions counter was existing, but was not used in driver
code - the required line had been added.

[rlibby]

Always nice to have minus diffs, assuming it still compiles :)

sys/dev/ena/ena.c

3407

Stray patch?

sys/dev/ena/ena.h

73

Can delete this too?

240

The leading whitespace looks funny in phabricator, can you double check it here?

[mk_semihalf.com]

sys/dev/ena/ena.c

3407

I wrote about this in the commit log. Should i split it into 2 patches?

[mk_semihalf.com]

Removed unused macro and additional whitespace