prevent rounding 0 if HWTIMER_SPEED > 1000000L

The current macros in hwtimer.h expect HWTIMER_SPEED to be < 1000000L, otherwise integer arithmetic will round the result down to 0.
Add a case to prevent that.

core/include/hwtimer.h

@@ -82,14 +82,22 @@ per second for the current architecture."
  * @param[in]   us number of microseconds
  * @return      kernel timer ticks
  */
-#define HWTIMER_TICKS(us) ((us) / (1000000L / HWTIMER_SPEED))
+#if HWTIMER_SPEED > 1000000L
+#define HWTIMER_TICKS(us)        ((us) * (HWTIMER_SPEED / 1000000L))
+#else
+#define HWTIMER_TICKS(us)        ((us) / (1000000L / HWTIMER_SPEED))
+#endif
 
 /**
  * @brief       Convert ticks to microseconds
  * @param[in]   ticks   number of ticks
  * @return      microseconds
  */
-#define HWTIMER_TICKS_TO_US(ticks) ((ticks) * (1000000L/HWTIMER_SPEED))
+#if HWTIMER_SPEED > 1000000L
+#define HWTIMER_TICKS_TO_US(ticks)        ((ticks) / (HWTIMER_SPEED / 1000000L))
+#else
+#define HWTIMER_TICKS_TO_US(ticks)        ((ticks) * (1000000L / HWTIMER_SPEED))
+#endif
 
 /**
  * @brief   Maximum hwtimer tick count (before overflow)
@@ -103,7 +111,11 @@ number of ticks countable on the current architecture."
 /**
  * @brief   microseconds before hwtimer overflow
  */
-#define HWTIMER_OVERFLOW_MICROS() (1000000L / HWTIMER_SPEED * HWTIMER_MAXTICKS)
+#if HWTIMER_SPEED > 1000000L
+#define HWTIMER_OVERFLOW_MICROS()        (HWTIMER_MAXTICKS / HWTIMER_SPEED * 1000000L)
+#else
+#define HWTIMER_OVERFLOW_MICROS()        (1000000L / HWTIMER_SPEED * HWTIMER_MAXTICKS)
+#endif
 
 typedef uint32_t timer_tick_t; /**< data type for hwtimer ticks */