sim: mn10300: fix LAST_TIMER_REG typo

The compiler pointed out that we're testing LAST_TIMER_REG and
LAST_COUNTER which are the same value ... and that's because we
set LAST_TIMER_REG to the wrong register.  Fix the typo.

sim/mn10300/dv-mn103tim.c

@@ -86,7 +86,7 @@ enum timer_register_types {
   TM6MDB,
   TM6CA,
   TM6CB,
-  LAST_TIMER_REG = TM6BC,
+  LAST_TIMER_REG = TM6CB,
 };