libata: quickly trigger SATA SPD down after debouncing failed

Debouncing failure is a good indicator of basic link problem.  Use
-EPIPE to indicate debouncing failure and make ata_eh_reset() invoke
sata_down_spd_limit() if the error occurs during reset.

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

drivers/ata/libata-core.c

@@ -3267,9 +3267,11 @@ int sata_phy_debounce(struct ata_port *ap, const unsigned long *params,
 		last = cur;
 		last_jiffies = jiffies;
 
-		/* check deadline */
+		/* Check deadline.  If debouncing failed, return
+		 * -EPIPE to tell upper layer to lower link speed.
+		 */
 		if (time_after(jiffies, deadline))
-			return -EBUSY;
+			return -EPIPE;
 	}
 }