libata-link: introduce ata_link

Introduce ata_link.  It abstracts PHY and sits between ata_port and
ata_device.  This new level of abstraction is necessary to support
SATA Port Multiplier, which basically adds a bunch of links (PHYs) to
a ATA host port.  Fields related to command execution, spd_limit and
EH are per-link and thus moved to ata_link.

This patch only defines the host link.  Multiple link handling will be
added later.  Also, a lot of ap->link derefences are added but many of
them will be removed as each part is converted to deal directly with
ata_link instead of ata_port.

This patch introduces no behavior change.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: James Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/pata_optidma.c

@@ -340,8 +340,8 @@ static int optidma_set_mode(struct ata_port *ap, struct ata_device **r_failed)
 		pci_read_config_byte(pdev, 0x43, &r);
 
 		r &= (0x0F << nybble);
-		r |= (optidma_make_bits43(&ap->device[0]) +
-		     (optidma_make_bits43(&ap->device[0]) << 2)) << nybble;
+		r |= (optidma_make_bits43(&ap->link.device[0]) +
+		     (optidma_make_bits43(&ap->link.device[0]) << 2)) << nybble;
 		pci_write_config_byte(pdev, 0x43, r);
 	}
 	return rc;