dpset: fix the new event mechnism breakage

Keep the compatibility for a while.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
test: fix integration tests: reflects 978f3e5c

  File "/home/openflow/git/osrg/ryu/ryu/tests/integrated/tester.py", line 167, in TestFlowBase
    @handler.set_ev_cls(dpset.EventDP, dpset.DPSET_EV_DISPATCHER)
AttributeError: 'module' object has no attribute 'DPSET_EV_DISPATCHER'

Signed-off-by: HIYAMA Manabu <hiyama.manabu@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>