fix fmu: lower polling timeout to 5ms when running as task

With the previous value of 20ms, we dropped RC input when not armed. The
result was that some RC channels jumped randomly between min & max.

src/drivers/px4fmu/fmu.cpp

@@ -1210,7 +1210,7 @@ PX4FMU::cycle()
 			_current_update_rate = 0;
 		}
 
-		int poll_timeout = 20;
+		int poll_timeout = 5; // needs to be small enough so that we don't miss RC input data
 
 		if (!_run_as_task) {
 			/*