6relayd: Fix DHCPv6-PD assignment logic

git-svn-id: svn://svn.openwrt.org/openwrt@36980 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: 6relayd: backport r36980

git-svn-id: svn://svn.openwrt.org/openwrt@36981 3c298f89-4303-0410-b956-a3cf2f4a3e73