target/s390x: fix build warning (gcc-12 -fsanitize=thread)

Found on debian stable.

../target/s390x/tcg/translate.c: In function ‘get_mem_index’:
../target/s390x/tcg/translate.c:398:1: error: control reaches end of non-void function [-Werror=return-type]
  398 | }

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20240814224132.897098-4-pierrick.bouvier@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

target/s390x/tcg/translate.c

@@ -392,7 +392,6 @@ static int get_mem_index(DisasContext *s)
         return MMU_HOME_IDX;
     default:
         g_assert_not_reached();
-        break;
     }
 #endif
 }