fix OFPMultipartRequest._serialize_stats_body

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
remove port_no argument of OFPPortDescStatsRequest.__init__

according to OF1.3/1.3.1 spec, the request body is empty.

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