atp870u: Rename is885() to atp_is()

Now that all the is* functions except is885() are gone, rename is885() to
atp_is() to avoid confusion. Don't know what "is" means, though...

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>