[SCSI] zfcp: Only increment req_id for successfully issued requests

Only increment the req_id for successfully issued requests. This
avoids some confusion when debugging issued fsf requests.

Acked-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>