include: linux: iio: Add missing kernel doc field

Fix kernel doc for the iio_dev_attr structure by adding its missing field.
Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/sysfs.h

@@ -18,7 +18,8 @@ struct iio_chan_spec;
  * struct iio_dev_attr - iio specific device attribute
  * @dev_attr:	underlying device attribute
  * @address:	associated register address
- * @l:		list head for maintaining list of dynamically created attrs.
+ * @l:		list head for maintaining list of dynamically created attrs
+ * @c:		specification for the underlying channel
  */
 struct iio_dev_attr {
 	struct device_attribute dev_attr;