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

@@ -49,14 +49,22 @@
  * @param[in] us number of microseconds
  * @return kernel timer ticks
  */
+#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)
@@ -69,7 +77,11 @@
 /**
  * @brief microseconds before hwtimer overflow
  */
+#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 */