ofproto: fix compile warning of ofproto/ofproto.c

ofproto: fix compile warning of ofproto/ofproto.c

This patch fixed the following compile warning:

ofproto/ofproto.c: In function 'handle_role_request':
ofproto/ofproto.c:3543: warning: format '%zu' expects type 'size_t', but argument 5 has type 'int'
ofproto/ofproto.c: In function 'handle_vendor':
ofproto/ofproto.c:3593: warning: format '%zu' expects type 'size_t', but argument 5 has type 'int'
ofproto/ofproto.c:3602: warning: format '%zu' expects type 'size_t', but argument 5 has type 'int'

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>