ofproto-dpif-upcall: Do not call dpif_handlers_set() when there is no
handler/revalidator configuration change.

dpif_handlers_set(), which dumps all vport from datapath, should not be
called constantly.

Found by inspection.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>