don't use Scala to Chisel implicit conversions outside of rocket

src/main/scala/rocket/frontend.scala

@@ -3,6 +3,7 @@ package rocket
 import Chisel._
 import uncore.tilelink._
 import util._
+import Chisel.ImplicitConversions._
 import cde.{Parameters, Field}
 
 class FrontendReq(implicit p: Parameters) extends CoreBundle()(p) {