evtimer: use conversion macro US_PER_MS

sys/evtimer/evtimer.c

@@ -94,7 +94,7 @@ static void _del_event_from_list(evtimer_t *evtimer, evtimer_event_t *event)
 
 static void _set_timer(xtimer_t *timer, uint32_t offset_ms)
 {
-    uint64_t offset_us = (uint64_t)offset_ms * 1000;
+    uint64_t offset_us = (uint64_t)offset_ms * US_PER_MS;
 
     DEBUG("evtimer: now=%" PRIu32 " us setting xtimer to %" PRIu32 ":%" PRIu32 " us\n",
           xtimer_now_usec(), (uint32_t)(offset_us >> 32), (uint32_t)(offset_us));