Update CFU immed field to use sext([31:24]) to match spec.

Signed-off-by: Tim Callahan <tcal@google.com>

src/main/scala/vexriscv/Riscv.scala

@@ -16,6 +16,7 @@ object Riscv{
   case class IMM(instruction  : Bits) extends Area{
     // immediates
     def i = instruction(31 downto 20)
+    def h = instruction(31 downto 24)
     def s = instruction(31 downto 25) ## instruction(11 downto 7)
     def b = instruction(31) ## instruction(7) ## instruction(30 downto 25) ## instruction(11 downto 8)
     def u = instruction(31 downto 12) ## U"x000"
@@ -24,6 +25,7 @@ object Riscv{
 
     // sign-extend immediates
     def i_sext = B((19 downto 0) -> i(11)) ## i
+    def h_sext = B((23 downto 0) -> h(7))  ## h
     def s_sext = B((19 downto 0) -> s(11)) ## s
     def b_sext = B((18 downto 0) -> b(11)) ## b ## False
     def j_sext = B((10 downto 0) -> j(19)) ## j ## False

src/main/scala/vexriscv/plugin/CfuPlugin.scala

@@ -187,7 +187,7 @@ class CfuPlugin(val stageCount : Int,
       if(p.CFU_INPUTS >= 1) bus.cmd.inputs(0) := input(RS1)
       if(p.CFU_INPUTS >= 2)  bus.cmd.inputs(1) := input(CFU_INPUT_2_KIND).mux(
         CfuPlugin.Input2Kind.RS -> input(RS2),
-        CfuPlugin.Input2Kind.IMM_I -> IMM(input(INSTRUCTION)).i_sext
+        CfuPlugin.Input2Kind.IMM_I -> IMM(input(INSTRUCTION)).h_sext
       )
     }