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
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30083