s390/cio: use DECLARE_WAIT_QUEUE_HEAD() for wait_queue

wait_queue_head_t can be initialized automatically with
DECLARE_WAIT_QUEUE_HEAD() rather than explicitly calling
init_waitqueue_head().

Signed-off-by: Shixin Liu <liushixin2@huawei.com>
Acked-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>