Fix gtimer on freertos

gtime tries to calculate from ticks to milliseconds by dividing by
`gfxMillisecondsToTicks(1)`. That'd be fine on systems where there's multiple
milliseconds per tick, but on freertos we count 10 ticks per millisecond, so
`gfxMillisecondsToTicks(1)` is 0, causing a division by zero later on.

For a 'proper' fix we'd need to implement gfxTicksToMilliseconds
on each backend, not just freertos

src/gos/gos_freertos.h

@@ -83,6 +83,7 @@ extern "C" {
 #define gfxYield()					taskYIELD()
 #define gfxSystemTicks()			xTaskGetTickCount()
 #define gfxMillisecondsToTicks(ms)	((systemticks_t)((ms) / portTICK_PERIOD_MS))
+#define gfxTicksToMilliseconds(ticks)	(ticks * portTICK_PERIOD_MS)
 #define gfxSystemLock()				taskENTER_CRITICAL()
 #define gfxSystemUnlock()			taskEXIT_CRITICAL()
 

src/gtimer/gtimer.c

@@ -22,7 +22,6 @@ static gfxMutex			mutex;
 static gfxThreadHandle	hThread = 0;
 static GTimer			*pTimerHead = 0;
 static gfxSem			waitsem;
-static systemticks_t	ticks2ms;
 static DECLARE_THREAD_STACK(waTimerThread, GTIMER_THREAD_WORKAREA_SIZE);
 
 /*===========================================================================*/
@@ -100,7 +99,7 @@ static DECLARE_THREAD_FUNCTION(GTimerThreadHandler, arg) {
 
 				// Find when we next need to wake up
 				if (!(pt->flags & GTIMER_FLG_INFINITE) && pt->when - tm < nxtTimeout)
-					nxtTimeout = (pt->when - tm)/ticks2ms;
+					nxtTimeout = gfxTicksToMilliseconds(pt->when - tm);
 				pt = pt->next;
 			} while(pt != pTimerHead);
 		}
@@ -116,7 +115,6 @@ void _gtimerInit(void)
 {
 	gfxSemInit(&waitsem, 0, 1);
 	gfxMutexInit(&mutex);
-	ticks2ms = gfxMillisecondsToTicks(1);
 }
 
 void _gtimerDeinit(void)