Fix setting AENQ group in ENA driver

With the current state of the AENQ handlers in the ENA driver, only
implemented handlers should be indicated.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: byenduri_gmail.com
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12872