scsi: stex: make S6flag static

This module specific flag can be made static as it does not need to be
in global scope.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Charles Chiou <charles.chiou@tw.promise.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>