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>