Use openolt adapter chart version 2.5.6

Change-Id: I7e04bbe4c1bc8d9e4c7c76000b8766a39c4f7d94
diff --git a/releases/voltha-2.4 b/releases/voltha-2.4
index eb7e730..00ce6d1 100644
--- a/releases/voltha-2.4
+++ b/releases/voltha-2.4
@@ -5,7 +5,7 @@
 
 export VOLTHA_CHART_VERSION=2.5.4
 export VOLTHA_BBSIM_CHART_VERSION=3.1.0
-export VOLTHA_ADAPTER_OPEN_OLT_CHART_VERSION=2.5.5
+export VOLTHA_ADAPTER_OPEN_OLT_CHART_VERSION=2.5.6
 export VOLTHA_ADAPTER_OPEN_ONU_CHART_VERSION=2.4.5
 export ONOS_CLASSIC_CHART_VERSION=0.1.1
 export RADIUS_CHART_VERSION=1.0.1