fix 77effb29 regression

The following commit broke ofp_attrs():

commit 77effb29a69eeb6d320dd9a7554924b2578c3c95
Author: Isaku Yamahata <yamahata@valinux.co.jp>
Date:   Mon May 27 18:15:27 2013 +0900

    ofproto/ofproto_parser: pylint

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
of1.3: various fixes

- OFPActionSetField
- OFPBucket parser
- OFPMeterBandStats

Signed-off-by: Shaun Crampton <Shaun.Crampton@metaswitch.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Ryu 2.0

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