ena: Bump driver version to v2.8.1

Changes since 2.8.0:

Bug Fixes:

• Fix LLQ normal width misconfiguration
• Check for errors when detaching children first, not last

Minor Changes:

• Remove \n from sysctl description

Approved by: cperciva (mentor)
MFC after: 3 days
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D50041