[SCSI] zfcp: Make queue_depth adjustable

zfcp did always set the queue_depth for SCSI devices to 32, not
allowing to change this. Introduce a kernel parameter zfcp.queue_depth
and the change_queue_depth callback to allow changing the queue_depth
when it is required.

Reviewed-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>