[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>

drivers/scsi/lpfc/lpfc_sli.c

@@ -16773,7 +16773,7 @@ lpfc_drain_txq(struct lpfc_hba *phba)
 {
 	LIST_HEAD(completions);
 	struct lpfc_sli_ring *pring = &phba->sli.ring[LPFC_ELS_RING];
-	struct lpfc_iocbq *piocbq = 0;
+	struct lpfc_iocbq *piocbq = NULL;
 	unsigned long iflags = 0;
 	char *fail_msg = NULL;
 	struct lpfc_sglq *sglq;