atp870u: Move chip-specific lines out of is880() and is885()

Move few chip-specifis lines out of is880() and is885() so they become
almost identical.

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>