xtimer: return after xtimer_spin() when within an ISR

sys/xtimer/xtimer.c

@@ -39,6 +39,7 @@ void _xtimer_sleep(uint32_t offset, uint32_t long_offset)
     if (irq_is_in()) {
         assert(!long_offset);
         xtimer_spin(offset);
+        return;
     }
 
     xtimer_t timer;