net: rpl: Neighbor data corrupted when receiving DAO message

The code was accessing wrong neighbor data when it received DAO
message. This corrupted nbr->iface pointer which was clearly seen
by "net nbr" shell command. The corruption then caused random
crashes or hangs when network interface via that pointer was
accessed.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>