ena: Update driver version to v2.8.4

Bug Fixes:

• Fix false 'missing TX completions' warnings due to timestamp race
• Put taskqueues into correct NUMA domain if !RSS

Minor Changes:

• Batch RX statistics updates
• Swap RX/TX completions cleanup order

Submitted by: Arthur Kiyanovski <akiyano@amazon.com>
MFC after: 2 weeks
Sponsored by: Amazon, Inc.
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D58242

(cherry picked from commit 605e699cd6ca4feae6c73c5c5ea8337054897116)