[SCSI] lpfc: use NULL instead of 0 for pointer

sparse says:
drivers/scsi/lpfc/lpfc_sli.c:16547:37: warning:
 Using plain integer as NULL pointer

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Acked-by: James Smart <james.smart@emulex.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>