service/ipfw: Silence warning on restart

Once the ipfw0 interface has been created, ifconfig(8) create will
throw a warning: "ifconfig: create: bad value" when trying to create
it again.

PR: 241013
Submitted by: Jose Luis Duran
Approved by: kp
Differential Revision: https://reviews.freebsd.org/D30083

(cherry picked from commit 5c4fe2ac81a5e05062266d684fb53b9faefd0d38)