Network: Set const properties where desired

We currently throw our own exceptions in these cases when we should just
be leveraging the generator and properly exposing our intention.

Change-Id: Ib16f95f1f04f7da8313c68245475ea87fec29e0b
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml b/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
index 012f87e..f45cac7 100644
--- a/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
+++ b/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
@@ -50,8 +50,8 @@
       type: array[string]
       description: >
           DNS servers that are currently in use on this interface.
-      errors:
-          - xyz::openbmc_project.Common.Error.NotAllowed
+      flags:
+          - readonly
     - name: StaticNameServers
       type: array[string]
       description: >