tests: fix test_hwtimer_spin

Actually test hwtimer_spin, not hwtimer_arch*

`hwtimer_wait` calls `hwtimer_spin` only for delays `<= 6`, that's why a
pure test_hwtimer_spin test should not pass values `>6`.
Also decrement delay in steps of 1.

tests/test_hwtimer_spin/main.c

 /*
- * Copyright (C) 2013 Ludwig Ortmann <ludwig.ortmann@fu-berlin.de>
+ * Copyright (C) 2014 Ludwig Ortmann <ludwig.ortmann@fu-berlin.de>
  *
  * This file is subject to the terms and conditions of the GNU Lesser General
  * Public License. See the file LICENSE in the top level directory for more
@@ -29,10 +29,10 @@
 
 int main(void)
 {
-    printf("when the race condition is hit, hwtimer will wait a very very long time...\n");
+    puts("This is just a functionality test for hwtimer_spin.");
 
     while (1) {
-        for (unsigned long i = 256; i; i = i >> 1) {
+        for (unsigned long i = 6; i > 0; i--) {
             printf("wait %lu\n", i);
             hwtimer_wait(i);
         }