media: cec: return -ENODEV instead of -ENXIO if unregistered

If the CEC device is unregistered, then an attempt to open
the device node should result in an -ENODEV error instead of
-ENXIO.

Document this as well in cec-func-open.rst.

This is consistent with the error code returned by other
file operations such as ioctl.

Signed-off-by: Hans Verkuil <[e-mail]>

Documentation/userspace-api/media/cec/cec-func-open.rst

···
 ``ENOMEM``
     Insufficient kernel memory was available.
 
-``ENXIO``
-    No device corresponding to this device special file exists.
+``ENODEV``
+    Device not found or was removed.

drivers/media/cec/core/cec-core.c

···
 	 */
 	mutex_lock(&devnode->lock);
 	/*
-	 * return ENXIO if the cec device has been removed
+	 * return ENODEV if the cec device has been removed
 	 * already or if it is not registered anymore.
 	 */
 	if (!devnode->registered) {
 		mutex_unlock(&devnode->lock);
-		return -ENXIO;
+		return -ENODEV;
 	}
 	/* and increase the device refcount */
 	get_device(&devnode->dev);