ofproto: Allow flow-dump matching on mpls tc using OpenFlow 1.2

OpenFlow 1.2 supports MPLS and should allow flow-dump requests
to match on the on the MPLS tc.

Without this change a request to do so will be downgraded to
NXT, which will fail because the osrg tree only permits
OpenFlow 1.2 connections.

Also add a test to exercise this case.

Signed-off-by: Simon Horman <horms@verge.net.au>