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 <[e-mail]>
Cc: Sebastian Ott <[e-mail]>
Cc: Peter Oberparleiter <[e-mail]>
Cc: [e-mail]
Cc: Martin Schwidefsky <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: [e-mail]
Signed-off-by: Sebastian Ott <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/include/asm/ccwgroup.h

···
  * @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 {