xhci: Return a USB 3.0 hub descriptor for USB3 roothub.

Return the correct xHCI roothub descriptor, based on whether the roothub
is marked as USB 3.0 or USB 2.0 in usb_hcd->bcdUSB.  Fill in
DeviceRemovable for the USB 2.0 and USB 3.0 roothub descriptors, using the
Device Removable bit in the port status and control registers.  xHCI is
the first host controller to actually properly set these bits (other hosts
say all devices are removable).

When userspace asks for a USB 2.0-style hub descriptor for the USB 3.0
roothub, stall the endpoint.  This is what real external USB 3.0 hubs do,
and we don't want to return a descriptor that userspace didn't ask for.

The USB core is already fixed to always ask for USB 3.0-style hub
descriptors.  Only usbfs (typically lsusb) will ask for the USB 2.0-style
hub descriptors.  This has already been fixed in usbutils version 0.91,
but the kernel needs to deal with older usbutils versions.

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