s390: fix new ccwgroup.h kernel-doc warning

Fix new s390 kernel-doc warning:

Warning(arch/s390/include/asm/ccwgroup.h:27): No description found for parameter 'ungroup_work'
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc:	Sebastian Ott <sebott@linux.vnet.ibm.com>
Cc:	Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Cc:	linux-s390@vger.kernel.org
Cc:	Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc:	Heiko Carstens <heiko.carstens@de.ibm.com>
Cc:	linux390@de.ibm.com
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/ccwgroup.h

@@ -10,6 +10,8 @@ struct ccw_driver;
  * @count: number of attached slave devices
  * @dev: embedded device structure
  * @cdev: variable number of slave devices, allocated as needed
+ * @ungroup_work: work to be done when a ccwgroup notifier has action
+ *	type %BUS_NOTIFY_UNBIND_DRIVER
  */
 struct ccwgroup_device {
 	enum {