scsi: lpfc: Set elsiocb contexts to NULL after freeing it

Set the elsiocb contexts to NULL after freeing as others depend on it.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Acked-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>