roundrobin: use nmigen zero-width signals

heavycomps/heavycomps/roundrobin.py

@@ -9,16 +9,12 @@ class RoundRobin:
 
     def elaborate(self, platform):
         m = Module()
-        n = self.n
-        if n > 1:
         with m.Switch(self.grant):
-                for i in range(n):
+            for i in range(self.n):
                 with m.Case(i):
                     with m.If(~self.request[i]):
-                            for j in reversed(range(i+1, i+n)):
-                                t = j % n
+                        for j in reversed(range(i+1, i+self.n)):
+                            t = j % self.n
                             with m.If(self.request[t]):
                                 m.d.sync += self.grant.eq(t)
-        else:
-            m.d.comb += self.grant.eq(0)
         return m