Do not initialize the adapter on MTU change when adapter status is down.
This fixes long-standing problems when changing settings of the adapter.
Discussed in:
https://lists.freebsd.org/pipermail/freebsd-net/2016-June/045509.html
Submitted by: arnaud.ysmal@stormshield.eu
Reviewed by: erj@freebsd.org
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7030