[ONOS-7176] Add DriverService dependency to GroupManager to ensure drivers are available when initial device events are received

Change-Id: Ibcc1c694ab95b68d607a76112f8f87d534e7f712
diff --git a/core/net/src/main/java/org/onosproject/net/group/impl/GroupManager.java b/core/net/src/main/java/org/onosproject/net/group/impl/GroupManager.java
index bc3c64d..9b82f5f 100644
--- a/core/net/src/main/java/org/onosproject/net/group/impl/GroupManager.java
+++ b/core/net/src/main/java/org/onosproject/net/group/impl/GroupManager.java
@@ -31,6 +31,7 @@
 import org.onosproject.net.device.DeviceEvent;
 import org.onosproject.net.device.DeviceListener;
 import org.onosproject.net.device.DeviceService;
+import org.onosproject.net.driver.DriverService;
 import org.onosproject.net.group.Group;
 import org.onosproject.net.group.GroupBuckets;
 import org.onosproject.net.group.GroupDescription;
@@ -87,6 +88,10 @@
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected DeviceService deviceService;
 
+    // Reference the DriverService to ensure the service is bound prior to initialization of the GroupDriverProvider
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected DriverService driverService;
+
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected ComponentConfigService cfgService;