improve LightShifterPlugin arbitration halt timings

src/main/scala/vexriscv/plugin/ShiftPlugins.scala

@@ -178,10 +178,12 @@ class LightShifterPlugin extends Plugin[VexRiscv]{
 
           when(done){
             isActive := False
-          } otherwise{
-            arbitration.haltItself := True
           }
         }
+
+        when(!done){
+          arbitration.haltItself := True
+        }
       }
       when(arbitration.removeIt){
         isActive := False