atp870u: Remove is880()

Now that is880() and is885() are almost identical (except for some cpu_relax()
calls and debug printks), remove is880() 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>