rpl: wrong length of DIO options

Currently, the DIO options `dodag conf` and `prefix info` are off by two
bytes in their `length` field. The RFC states, that the length field
should not include the option `type` field and the `length` field (two bytes).

For Prefix Info Option: Option Length: 30 (RFC 6550, P.61)
For Dodag Conf  Option: Option Length: 14 (RFC 6550, P.52)

Wireshark complains about DIOs as malformed packets, otherwise.
Can be reproduced by running the rpl_udp example and logging the DIOs
via wireshark.