libata: don't whine if ->prereset() returns -ENOENT

->prereset() returns -ENOENT to tell libata that the port is empty and
reset sequencing should be stopped.  This is not an error condition.
Update ata_eh_reset() such that it sets device classes to ATA_DEV_NONE
and return success in on -ENOENT.  This makes spurious error message
go away.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-eh.c

@@ -1625,8 +1625,14 @@ static int ata_eh_reset(struct ata_port *ap, int classify,
 		rc = prereset(ap);
 		if (rc) {
 			if (rc == -ENOENT) {
-				ata_port_printk(ap, KERN_DEBUG, "port disabled. ignoring.\n");
+				ata_port_printk(ap, KERN_DEBUG,
+						"port disabled. ignoring.\n");
 				ap->eh_context.i.action &= ~ATA_EH_RESET_MASK;
+
+				for (i = 0; i < ATA_MAX_DEVICES; i++)
+					classes[i] = ATA_DEV_NONE;
+
+				rc = 0;
 			} else
 				ata_port_printk(ap, KERN_ERR,
 					"prereset failed (errno=%d)\n", rc);