[SCSI] st: Use static class attributes

st currently sets up and tears down class attributes manually for
every tape drive in the system. This patch uses a statically defined
class with class attributes to let the device core do it for us.

Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>