btrfs: define compression levels statically

The maximum and default levels do not change and can be defined
directly. The set_level callback was a temporary solution and will be
removed.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>