atp870u: Convert is880() to use wrappers

Subtract 0x40 to use _io access wrappers.  Now it's obvious that is870()
and is880() are very similar.

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>