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

src/main/scala/rocket/fpu.scala

@@ -5,6 +5,7 @@ package rocket
 import Chisel._
 import Instructions._
 import util._
+import Chisel.ImplicitConversions._
 import FPConstants._
 import uncore.constants.MemoryOpConstants._
 import cde.{Parameters, Field}