atp870u: Introduce is880(), is885() and remove dev_id

Introduce chip type inline functions to simplify code, allowing to delete
dev_id from struct atp_unit.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>