scsi: cxlflash: fix assignment of the backend operations

commit cd43c221 ("scsi: cxlflash: Isolate external module
dependencies") introduced the use of ifdefs to avoid compilation errors
when one of the possible backend driver, CXL or OCXL, is not compiled.

Unfortunately, the wrong defines are used and the backend ops are never
assigned, leading to a kernel crash in any case when the cxlflash module is
loaded.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>