staging: comedi: usbdux: fix usbdux_pwm_cancel()

Add the missing down/up of the semaphore to prevent other commands
from being issued to the usb device while the pwn is being stopped.

Rename the local variable used for the private data pointer to the
comedi "norm".

Make sure to check that usbdux_pwm_stop() was successful before
sending command to the usb device to stop the pwm.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/usbdux.c

@@ -1430,17 +1430,25 @@ static int usbdux_pwm_stop(struct comedi_device *dev, int do_unlink)
 	return ret;
 }
 
-/* force unlink - is called by comedi */
 static int usbdux_pwm_cancel(struct comedi_device *dev,
 			     struct comedi_subdevice *s)
 {
-	struct usbdux_private *this_usbduxsub = dev->private;
-	int res = 0;
+	struct usbdux_private *devpriv = dev->private;
+	int ret;
+
+	down(&devpriv->sem);
 
 	/* unlink only if it is really running */
-	res = usbdux_pwm_stop(dev, this_usbduxsub->pwm_cmd_running);
+	ret = usbdux_pwm_stop(dev, devpriv->pwm_cmd_running);
+	if (ret)
+		goto pwm_cancel_exit;
+
+	ret = send_dux_commands(dev, SENDPWMOFF);
 
-	return send_dux_commands(dev, SENDPWMOFF);
+pwm_cancel_exit:
+	up(&devpriv->sem);
+
+	return ret;
 }
 
 static void usbduxsub_pwm_irq(struct urb *urb)