riscv64,difftest,ref: do not check csr

* we may enable CSR checking in the future

src/isa/riscv64/difftest/ref.c

@@ -4,6 +4,7 @@
 #include "difftest.h"
 
 static void csr_prepare() {
+  return;
   cpu.mstatus = mstatus->val;
   cpu.mcause  = mcause->val;
   cpu.mepc    = mepc->val;
@@ -16,6 +17,7 @@ static void csr_prepare() {
 }
 
 static void csr_writeback() {
+  return;
   mstatus->val = cpu.mstatus;
   mcause ->val = cpu.mcause ;
   mepc   ->val = cpu.mepc   ;