HAL_PX4: always use the hrt semaphore based delay

the up_udelay() could cause too much timing jitter

libraries/AP_HAL_PX4/Scheduler.cpp

@@ -102,6 +102,7 @@ void PX4Scheduler::delay_microseconds_semaphore(uint16_t usec)
     sem_t wait_semaphore;
     struct hrt_call wait_call;
     sem_init(&wait_semaphore, 0, 0);
+    memset(&wait_call, 0, sizeof(wait_call));
     hrt_call_after(&wait_call, usec, (hrt_callout)sem_post, &wait_semaphore);
     sem_wait(&wait_semaphore);
 }
@@ -109,16 +110,7 @@ void PX4Scheduler::delay_microseconds_semaphore(uint16_t usec)
 void PX4Scheduler::delay_microseconds(uint16_t usec) 
 {
     perf_begin(_perf_delay);
-    if (usec >= 500) {
-        delay_microseconds_semaphore(usec);
-        perf_end(_perf_delay);
-        return;
-    }
-	uint64_t start = micros64();
-    uint64_t dt;
-	while ((dt=(micros64() - start)) < usec) {
-		up_udelay(usec - dt);
-	}
+    delay_microseconds_semaphore(usec);
     perf_end(_perf_delay);
 }