Cover ENA driver code with branch predictioning statements

The driver was using it in only few places, so the rest of the code
was covered with those statement.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: rlibby
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12861