device: fix intersecting IPv6 configurations

If the link is down we shouldn't drop the link-local address from
configuration as it wasn't removed by user but by kernel.
12 jobs for bg/restore-ip6-addr-link-up-rh1548237 in 26 minutes and 29 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #287772
allowed to fail
checkpatch

00:01:28

passed #287779
t_debian:9

00:15:26

passed #287781
t_debian:sid

00:12:05

manual #287780
allowed to fail manual
t_debian:testing
passed #287773
t_fedora:28

00:17:54

passed #287774
t_fedora:29

00:26:28

passed #287775
t_fedora:30

00:20:02

manual #287776
allowed to fail manual
t_fedora:rawhide
passed #287777
t_ubuntu:16.04

00:17:13

passed #287778
t_ubuntu:18.04

00:17:15

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:59:08