scsi: scsi_debug: Fix uninitialized variable use

scsi: scsi_debug: Fix uninitialized variable use

It appears that a typo has made it into the newly added code

drivers/scsi/scsi_debug.c:3035:3: error: variable 'len' is uninitialized when used here [-Werror,-Wuninitialized]
 3035 |                 len += resp_compression_m_pg(ap, pcontrol, target, devip->tape_dce);
      |                 ^~~

Replace the '+=' with the intended '=' here.

Fixes: 568354b24c7d ("scsi: scsi_debug: Add compression mode page for tapes")
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250225095651.2636811-1-arnd@kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>