Fix compilation warnings when building ENA driver with gcc compiler

The gcc compiler is more sensitive when variable is having an value
assigned, but it is not used anywhere further.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: rlibby
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12866