device: set IPv6 token only when necessary

Setting the IPv6 token triggers a new router solicitation from kernel
and so we should avoid when not strictly necessary.

https://mail.gnome.org/archives/networkmanager-list/2019-May/msg00004.html
#179
14 jobs for bg/ipv6-token in 37 minutes and 20 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #380047
allowed to fail
checkpatch

00:01:32

manual #380052
allowed to fail manual
t_centos:7.5.1804
passed #380053
t_centos:7.6.1810

00:33:30

passed #380056
t_debian:9

00:20:08

passed #380058
t_debian:sid

00:19:58

manual #380057
allowed to fail manual
t_debian:testing
passed #380048
t_fedora:28

00:31:36

passed #380049
t_fedora:29

00:37:18

passed #380050
t_fedora:30

00:30:21

manual #380051
allowed to fail manual
t_fedora:rawhide
passed #380054
t_ubuntu:16.04

00:21:07

passed #380055
t_ubuntu:18.04

00:16:02

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:20:28