ofproto-dpif: Remove datapath flows of deleted ports.

Commit acf608 (ofproto-dpif: Use a single underlying datapath across
multiple bridges.) causes datapath flows from deleted ports to not be
removed.  The issue is that the code that bulk deletes old flows doesn't
know the datapath port number that makes up the datapath flow
definition.  This commit keeps track of the datapath port in the facet
for use when the datapath flow eventually needs to be removed.

Signed-off-by: Justin Pettit <jpettit@nicira.com>