libata: forbid port runtime pm by default, fixing regression

Forbid port runtime pm by default because it has known hotplug issue.
User can allow it by, for example

echo auto > /sys/devices/pci0000:00/0000:00:1f.2/ata2/power/control
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-transport.c

@@ -294,6 +294,7 @@ int ata_tport_add(struct device *parent,
 	device_enable_async_suspend(dev);
 	pm_runtime_set_active(dev);
 	pm_runtime_enable(dev);
+	pm_runtime_forbid(dev);
 
 	transport_add_device(dev);
 	transport_configure_device(dev);