libata: mask off DET when restoring SControl for detach

libata restores SControl on detach; however, trying to restore
non-zero DET can cause undeterministic behavior including PMP device
going offline till power cycling.  Mask off DET when restoring
SControl.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-core.c

@@ -5936,7 +5936,7 @@ static void ata_port_detach(struct ata_port *ap)
 	 * to us.  Restore SControl and disable all existing devices.
 	 */
 	__ata_port_for_each_link(link, ap) {
-		sata_scr_write(link, SCR_CONTROL, link->saved_scontrol);
+		sata_scr_write(link, SCR_CONTROL, link->saved_scontrol & 0xff0);
 		ata_link_for_each_dev(dev, link)
 			ata_dev_disable(dev);
 	}