airspeed_selector: fix timeout check

I think this check was the wrong way round which meant that we would end
up with an unsigned int overflow and trigger a reset.

src/modules/airspeed_selector/airspeed_selector_main.cpp

@@ -357,7 +357,7 @@ AirspeedModule::Run()
 
 				_time_last_airspeed_update[i] = _time_now_usec;
 
-			} else if (_time_last_airspeed_update[i] - _time_now_usec > 1_s) {
+			} else if (_time_now_usec - _time_last_airspeed_update[i] > 1_s) {
 				// declare airspeed invalid if more then 1s since last raw airspeed update
 				_airspeed_validator[i].reset_airspeed_to_invalid(_time_now_usec);