scsi: scsi_debug: Rework subpage code error handling

Move the subpage code checks into the switch statement to make it easier
to add support for new page code / subpage code combinations.

Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240130214911.1863909-16-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>