drv_hrt: allow timedifferences across timestamp wrap again

reverting a small part of
4a55393

src/drivers/drv_hrt.h

@@ -162,16 +162,7 @@ static inline void abstime_to_ts(struct timespec *ts, hrt_abstime abstime)
  */
 static inline hrt_abstime hrt_elapsed_time(const hrt_abstime *then)
 {
-	hrt_abstime now = hrt_absolute_time();
-
-	// Cannot allow a negative elapsed time as this would appear
-	// to be a huge positive elapsed time when represented as an
-	// unsigned value!
-	if (*then > now) {
-		return 0;
-	}
-
-	return now - *then;
+	return hrt_absolute_time() - *then;
 }
 
 /**