scsi: zfcp: ERP thread setup kdoc update

zfcp_erp_thread_setup() update complements v2.6.32 commit 347c6a96
("[SCSI] zfcp: Use kthread API for zfcp erp thread").

Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>