atp870u: Add channel parameter to is870() and is880()

Add channel parameter to is870() and is880() functions to simplify comparing
them with is885().
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>