Fix bug in GCU vlanintf_validator (#2765)

What I did
Fix the bug in vlanintf_validator

How I did it
ret will be 0 if success. Fix the statement.

How to verify it
E2E test

Author: wen587

generic_config_updater/services_validator.py

@@ -119,6 +119,6 @@ def vlanintf_validator(old_config, upd_config, keys):
     for key in deleted_keys:
         iface, iface_ip = key
         rc = os.system(f"ip neigh flush dev {iface} {iface_ip}")
-        if not rc:
+        if rc:
             return False
     return True