s390/cio: use DECLARE_WAIT_QUEUE_HEAD for static work_queue_head_t

Use DECLARE_WAIT_QUEUE_HEAD to declare and statically initialize the
work_queue_head_t.

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>