s390/zfcp: use qdio buffer helpers

Use qdio buffer helpers to manage the buffers used for the request
and response queues.

No functional change.

Reviewed-by: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>