Fix the folloing problem in MTIPv6Src, MTIPv6Src, MTIPv6Dst, and
MTIPv6NdTarget's parsers:
File "/home/openflow/ryu/ryu/ofproto/ofproto_v1_2_parser.py", line 1705, in parser
field = OFPMatchField.parser(buf, offset)
File "/home/openflow/ryu/ryu/ofproto/ofproto_v1_2_parser.py", line 1934, in parser
field = cls_.field_parser(header, buf, offset)
File "/home/openflow/ryu/ryu/ofproto/ofproto_v1_2_parser.py", line 1949, in field_parser
(value,) = struct.unpack_from(cls.pack_str, buf, offset + 4)
ValueError: too many values to unpack
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>