sg: restore command permission for TYPE_SCANNER

sg allowed any command for TYPE_SCANNER. The cmd_filter patchset
doesn't. We can't change sg's permission since it might break the
existing software.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>