scsi: zfcp: improve kdoc for return of zfcp_status_read_refill()

Complements

v2.6.35 commit 64deb6ef ("[SCSI] zfcp: Use status_read_buf_num
provided by FCP channel") which replaced the hardcoded 16 with a
variable value

Also complements already existing fixups for above commit

v2.6.35 commit 8d88cf3f ("[SCSI] zfcp: Update status read mempool")
v3.10   commit 9edf7d75

 ("[SCSI] zfcp: status read buffers on first adapter open with link down")

Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>