Restore r302384 that was dropped when r303816 updated the driver to 1.6.6.-k

Original log:
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

Reported by: Franco Fichtner <franco@opnsense.org>
MFH: 2 days