staging: comedi: restrict comedi_set_hw_dev() usage

Don't allow comedi drivers to change `dev->hw_dev` using
`comedi_set_hw_dev()` if it's already been set.  Return `-EEXIST` in
that case.

`dev->hw_dev` needs to be set to NULL by the core during clean-up of the
comedi device, so add a local function `comedi_clear_hw_dev()` to do
that.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers.c

@@ -45,14 +45,21 @@ struct comedi_driver *comedi_drivers;
 
 int comedi_set_hw_dev(struct comedi_device *dev, struct device *hw_dev)
 {
-	struct device *old_hw_dev = dev->hw_dev;
-
+	if (hw_dev == dev->hw_dev)
+		return 0;
+	if (dev->hw_dev != NULL)
+		return -EEXIST;
 	dev->hw_dev = get_device(hw_dev);
-	put_device(old_hw_dev);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(comedi_set_hw_dev);
 
+static void comedi_clear_hw_dev(struct comedi_device *dev)
+{
+	put_device(dev->hw_dev);
+	dev->hw_dev = NULL;
+}
+
 int comedi_alloc_subdevices(struct comedi_device *dev, int num_subdevices)
 {
 	struct comedi_subdevice *s;
@@ -108,7 +115,7 @@ static void cleanup_device(struct comedi_device *dev)
 	dev->write_subdev = NULL;
 	dev->open = NULL;
 	dev->close = NULL;
-	comedi_set_hw_dev(dev, NULL);
+	comedi_clear_hw_dev(dev);
 }
 
 static void __comedi_device_detach(struct comedi_device *dev)