use toImmutableSet converting Stream to Set

Change-Id: Ifc6eac3bfd524fbf076a076bb73e5b2e51706066
diff --git a/drivers/optical/src/main/java/org/onosproject/driver/optical/query/CalientLambdaQuery.java b/drivers/optical/src/main/java/org/onosproject/driver/optical/query/CalientLambdaQuery.java
index 05d7f7e..3f0bf48 100644
--- a/drivers/optical/src/main/java/org/onosproject/driver/optical/query/CalientLambdaQuery.java
+++ b/drivers/optical/src/main/java/org/onosproject/driver/optical/query/CalientLambdaQuery.java
@@ -15,6 +15,7 @@
  */
 package org.onosproject.driver.optical.query;
 
+import org.onlab.util.GuavaCollectors;
 import org.onlab.util.Spectrum;
 import org.onosproject.net.ChannelSpacing;
 import org.onosproject.net.GridType;
@@ -24,7 +25,6 @@
 import org.onosproject.net.driver.AbstractHandlerBehaviour;
 
 import java.util.Set;
-import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 
 /**
@@ -48,6 +48,6 @@
         return IntStream.rangeClosed((int) startSpacingMultiplier, (int) stopSpacingMultiplier)
                 .filter(i -> i % 2 == 1)
                 .mapToObj(i -> new OchSignal(GridType.FLEX, ChannelSpacing.CHL_6P25GHZ, i, 1))
-                .collect(Collectors.toSet());
+                .collect(GuavaCollectors.toImmutableSet());
     }
 }