Ensure that enum doesn't contain type definition

Adds check into enum validate pass.

Test: hidl_test
Change-Id: I436d5642e2bf52a7658828d947d84d063b64204b
diff --git a/EnumType.cpp b/EnumType.cpp
index 508328f..38f74ba 100644
--- a/EnumType.cpp
+++ b/EnumType.cpp
@@ -80,6 +80,8 @@
 }
 
 status_t EnumType::validate() const {
+    CHECK(getSubTypes().empty());
+
     status_t err = mStorageType->callForReference(&Type::validate);
     if (err != OK) return err;