of1.3: fix OFPort parser

s/length/_length/

Trace:
[0x1000082e5f694c00] ('143.0.1.211', 56538): OFPGetConfigReply received:
        flags=NORMAL miss_send_len=1518
Error in the datapath 1000082e5f694c00 from ('143.0.1.211', 56538)
hub: uncaught exception: Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/lib/hub.py", line 48, in _launch
    func(*args, **kwargs)
 File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/controller/controller.py", line 312, in datapath_connection_factory
    datapath.serve()
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/controller/controller.py", line 232, in serve
    self._recv_loop()
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/controller/controller.py", line 97, in deactivate
    method(self)
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/controller/controller.py", line 165, in _recv_loop
    version, msg_type, msg_len, xid, buf)
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/ofproto/ofproto_parser.py", line 54, in msg
    return msg_parser(datapath, version, msg_type, msg_len, xid, buf)
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/ofproto/ofproto_v1_3_parser.py", line 50, in msg_parser
    return parser(datapath, version, msg_type, msg_len, xid, buf)
  File "/usr/local/lib/python2.6/dist-packages/ryu-2.1-py2.6.egg/ryu/ofproto/ofproto_v1_3_parser.py", line 2192, in parser
    offset += b._length
AttributeError: 'OFPPort' object has no attribute '_length'

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>