ofp-util: Use 32bit port for Open Flow 1.1 & 1.2 Port Mod Request

When encoding Open Flow 1.1 & 1.2 Port Mod Request messages
the port number should be converted to 32bits using
ofputil_port_to_ofp11() rather than htonl(). This ensures
that port numbers in the reserved range are translated correctly.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>