Update ENA version to v2.1.0

In this release the netmap support was introduced.

Moreover, it is also now possible to use the LLQ mode of the driver on
the arm64 AWS instances (A1 type).

Differential Revision: https://reviews.freebsd.org/D21938
Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.