scsi: cxlflash: Fix an error handling path in 'cxlflash_disk_attach()'

'rc' is known to be 0 at this point.  If 'create_context()' fails,
returns -ENOMEM instead of 0 which means success.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>