SEBA-345: Upgrade to current gRPC and Protobuf libs

Change-Id: I7167a0a490e38cb40b8bd37da81bf78a8e3a86ea
diff --git a/ponsim/grpc_server.py b/ponsim/grpc_server.py
index 9165433..3185c9e 100644
--- a/ponsim/grpc_server.py
+++ b/ponsim/grpc_server.py
@@ -19,9 +19,9 @@
 from concurrent import futures
 
 from voltha.protos import third_party
-from voltha.protos.ponsim_pb2 import add_PonSimServicer_to_server
-from voltha.protos.ponsim_pb2 import add_XPonSimServicer_to_server
-from voltha.adapters.asfvolt16_olt.protos.bal_pb2 import add_BalServicer_to_server
+from voltha.protos.ponsim_pb2_grpc import add_PonSimServicer_to_server
+from voltha.protos.ponsim_pb2_grpc import add_XPonSimServicer_to_server
+from voltha.adapters.asfvolt16_olt.protos.bal_pb2_grpc import add_BalServicer_to_server
 from ponsim_servicer import FlowUpdateHandler, XPonHandler
 from bal_servicer import BalHandler