Fix ENA keep-alive timeout due to prolonged reset

When the ENA_FLAG_DEVICE_RUNNING flag is disabled, the AENQ handlers
aren't executed. To fix that, the watchdog timestamp should be updated
just before enabling the watchdog.

Timer service was always being enabled, even if the device wasn't up
before the reset. That shouldn't happen, as the timer service is being
executed only for working interface.

Differential Revision: https://reviews.freebsd.org/D21932
Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.