Change order of ifp release on ENA detach

In rare case, when the ifconfig is called just before kldunload, it is
possible, that ena_up routine will be called after queue locks are
released.

To prevent that, ifp is detached before the last ena_down is called and
further, the ifp is freed at the end of the function.

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