alpha: add missing 'break', spotted by clang analyzer

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/target-alpha/helper.c b/target-alpha/helper.c
index 46335cd..9a039cb 100644
--- a/target-alpha/helper.c
+++ b/target-alpha/helper.c
@@ -466,6 +466,7 @@
             env->ipr[IPR_SYSPTBR] = val;
         else
             ret = -1;
+        break;
     case IPR_TBCHK:
         /* Read-only */
         ret = -1;