Mark the ENA driver as epoch ready

Recent changes to the epoch requires driver to notify that they knows
epoch in order to prevent input packet function to enter epoch each
time the packet is received.

ENA is using NET_TASK for handling Rx, so it's entering epoch
automatically whenever this task is being executed.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.