Fix regression introduced on 272446r.

lagg(4) supports the protocol none, where it disables any traffic without
disabling the lagg(4) interface itself.

PR: 206921
Submitted by: Pushkar Kothavade <pushkarbk@gmail.com>
Reviewed by: rpokala
Approved by: bapt (mentor)
MFC after: 3 weeks
Sponsored by: gandi.net
Differential Revision: https://reviews.freebsd.org/D5076