[VOL-3548] Differentiate between OLT gRPC channel disconnect and device reboot

- Return the previously_connected state flag in response to GetDeviceInfo to VOLTHA

Change-Id: I3a39eccdda386658226c6379a4815900b808795d
diff --git a/Makefile b/Makefile
index a689a37..ecd0b52 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@
 
 ## Variables
 OPENOLTDEVICE     ?= asfvolt16
-OPENOLT_PROTO_VER ?= v4.0.14
+OPENOLT_PROTO_VER ?= v4.0.15
 
 DOCKER                     ?= docker
 DOCKER_REGISTRY            ?=