odhcp6c: work around RIOs matching PIOs as sent by some apple routers

Thanks to Mikael Abrahamsson for reporting.

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45941 3c298f89-4303-0410-b956-a3cf2f4a3e73