atp870u: Remove is870()

Now that is885() supports everything from is870() and the rest of the code
is almost identical, remove is870() and use is885() instead.
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>