USB/xhci: Enable remote wakeup for USB3 devices.

When the USB 3.0 hub support went in, I disabled selective suspend for
all external USB 3.0 hubs because they used a different mechanism to
enable remote wakeup.  In fact, other USB 3.0 devices that could signal
remote wakeup would have been prevented from going into suspend because
they would have stalled the SetFeature Device Remote Wakeup request.

This patch adds support for the USB 3.0 way of enabling remote wake up
(with a SetFeature Function Suspend request), and enables selective
suspend for all hubs during hub_probe.  It assumes that all USB 3.0 have
only one "function" as defined by the interface association descriptor,
which is true of all the USB 3.0 devices I've seen so far.  FIXME if
that turns out to change later.

After a device signals a remote wakeup, it is supposed to send a Device
Notification packet to the host controller, signaling which function
sent the remote wakeup.  The host can then put any other functions back
into function suspend.  Since we don't have support for function suspend
(and no devices currently support it), we'll just assume the hub
function will resume the device properly when it received the port
status change notification, and simply ignore any device notification
events from the xHCI host controller.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>