EthernetInterface: Change LinkUp property to readonly

A const property never changes during the lifetime of the object it
belongs to. But in phosphor-networkd, linkUp will be changed by the
service itself. It should be readonly instead of const.

Change-Id: I42a3c96edd68f4a2dbd81e2ab84583d6b3239c6d
Signed-off-by: Jiaqing Zhao <jiaqing.zhao@intel.com>
diff --git a/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml b/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
index 94ff988..f382ee1 100644
--- a/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
+++ b/yaml/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
@@ -78,7 +78,7 @@
     - name: LinkUp
       type: boolean
       flags:
-          - const
+          - readonly
       description: >
           This property reports the link status for the NIC. The read-only
           boolean value is TRUE when the network cable is inserted, or there