Avoid setting UNI status to disabled if it's already down

Change-Id: Id2ca815ee46ed623096af93e56184d5c77f96d86
diff --git a/internal/bbsim/devices/uni_port.go b/internal/bbsim/devices/uni_port.go
index da16dcc..eeda028 100644
--- a/internal/bbsim/devices/uni_port.go
+++ b/internal/bbsim/devices/uni_port.go
@@ -208,6 +208,9 @@
 }
 
 func (u *UniPort) Disable() error {
+	if u.OperState.Is(UniStateDown) {
+		return nil
+	}
 	return u.OperState.Event(uniTxDisable)
 }