hwmon: (iio_hwmon) Fix documentation of struct iio_hwmon_state

Sparse reports:

drivers/hwmon/iio_hwmon.c:36: warning:
	No description found for parameter 'groups'
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

drivers/hwmon/iio_hwmon.c

@@ -24,6 +24,7 @@
  * @num_channels:	number of channels in channels (saves counting twice)
  * @hwmon_dev:		associated hwmon device
  * @attr_group:		the group of attributes
+ * @groups:		null terminated array of attribute groups
  * @attrs:		null terminated array of attribute pointers.
  */
 struct iio_hwmon_state {