fix D$ writeback bug

I swear I did this last week... perhaps I am finally losing it!

rocket/src/main/scala/util.scala

@@ -7,6 +7,7 @@ object Util
   implicit def intToUFix(x: Int): UFix = UFix(x)
   implicit def intToBoolean(x: Int): Boolean = if (x != 0) true else false
   implicit def booleanToInt(x: Boolean): Int = if (x) 1 else 0
+  implicit def booleanToBool(x: Boolean): Bits = Bool(x)
 
   implicit def wcToUFix(c: WideCounter): UFix = c.value
 }