ofp-msgs: Update for OpenFlow 1.1 & 1.2 Port Desc Stats

ofp-msgs: Update for OpenFlow 1.1 & 1.2 Port Desc Stats

* Split OFPRAW_OFPST_PORT_DESC_REPLY into OpenFlow 1.0
  and OpenFlow 1.1+ versions.
* Allow OFPRAW_OFPST_PORT_DESC_REQUEST to be used for
  OpenFlow 1.0+ instead of only OpenFlow 1.0.

This appears to be necessary and sufficient to allow encoding of Port Desc
Stats reply messages and decoding of Port Desc Stats request messages. Or
in other words both the client and server side of ovs-ofctl dump-ports-desc

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