atp870u: Add remaining 870 support to is885()

Add remaining 870 support to is885():
 - different synw, no synuw
 - synu[4] = 0x0c
 - atp_writeb_io(dev, c, 0x04, 0x00); instead of
   atp_writeb_io(dev, c, 0x14, 0x00); (isn't that a bug?)
 - atp_writeb_io(dev, c, 0x14, 0xff); instead of
   atp_writeb_io(dev, c, 0x14, 0x06);
 - different mbuf[3] and mbuf[4] checks

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>