Reduce log level in "ensure_device_is_ready"

If the device is not ready, the method should inform about this
event. The code calling this method, if needed, can write a higher
log message.

Change-Id: Ib7c5ba736f6e4ccc88df665faeef304c176a24e7
Closes-Bug: #1896920
(cherry picked from commit b1f1c8aa50)

neutron/agent/linux/ip_lib.py

@@ -969,8 +969,8 @@ def ensure_device_is_ready(device_name, namespace=None):
         # Ensure the device has a MAC address and is up, even if it is already
         # up.
         if not dev.link.exists or not dev.link.address:
-            LOG.error("Device %s cannot be used as it has no MAC "
-                      "address", device_name)
+            LOG.info("Device %s cannot be used as it has no MAC "
+                     "address", device_name)
             return False
         dev.link.set_up()
     except RuntimeError: