Add a new taskqueue (device specific, not net80211 ic->tq); use it for
device restart.

(Committers note - once scan overhaul and a few other things have been
fixed in net80211 to not block things in the taskqueue, this can disappear
and the device specific taskqueues in other drivers can also go away.)

PR: kern/197143
Submitted by: Andriy Voskoboinyk <s3erios@gmail.com>