I2C airspeed driver needs 2 retries

this prevents I2C transfer errors every few seconds with the
meas_airspeed driver

src/drivers/airspeed/airspeed.cpp

@@ -157,7 +157,7 @@ Airspeed::probe()
 	*/
 	_retries = 4;
 	int ret = measure();
-	_retries = 0;
+	_retries = 2;
 	return ret;
 }