scsi: lpfc: Return NULL rather than a plain 0 integer

Function lpfc_sli4_perform_vport_cvl() returns a pointer to struct
lpfc_nodelist so returning a plain 0 integer isn't good practice.  Fix this
by returning a NULL instead.

Link: https://lore.kernel.org/r/20210925224113.183040-1-colin.king@canonical.com


Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>