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>
diff --git a/drivers/ata/sata_via.c b/drivers/ata/sata_via.c
index 57fd30d..78a6833 100644
--- a/drivers/ata/sata_via.c
+++ b/drivers/ata/sata_via.c
@@ -296,7 +296,7 @@
  */
 static int vt6420_prereset(struct ata_port *ap, unsigned long deadline)
 {
-	struct ata_eh_context *ehc = &ap->eh_context;
+	struct ata_eh_context *ehc = &ap->link.eh_context;
 	unsigned long timeout = jiffies + (HZ * 5);
 	u32 sstatus, scontrol;
 	int online;