[SCSI] zfcp: fix return code of zfcp_scsi_slave_alloc

Change return code in slave_alloc to avoid irritating error message from
scsi_alloc_sdev() when scsi stack tries target scan.

Signed-off-by: Andreas Herrmann <aherrman@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>