px4io: allow set of output rates above 400 and below 50

let the IO board decide if the rate is reasonable, and limit it there

this fixes the rates on ArduCopter, which try for 490

src/drivers/px4io/px4io.cpp

@@ -1336,11 +1336,7 @@ PX4IO::ioctl(file *filep, int cmd, unsigned long arg)
 
 	case PWM_SERVO_SET_UPDATE_RATE:
 		/* set the requested alternate rate */
-		if ((arg >= 50) && (arg <= 400)) {	/* TODO: we could go higher for e.g. TurboPWM */
-			ret = io_reg_set(PX4IO_PAGE_SETUP, PX4IO_P_SETUP_PWM_ALTRATE, arg);
-		} else {
-			ret = -EINVAL;
-		}
+		ret = io_reg_set(PX4IO_PAGE_SETUP, PX4IO_P_SETUP_PWM_ALTRATE, arg);
 		break;
 
 	case PWM_SERVO_SELECT_UPDATE_RATE: {