dhcpv6_client: use milliseconds rather than microseconds as base unit
dhcpv6_client: use milliseconds rather than microseconds as base unit

kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,ant9000/RIOT