xhci: Modify check for TT info.

Commit d199c96d by Alan Stern ensured that low speed and full speed
devices below a high speed hub without a transaction translator (TT) would
never get enumerated.  Simplify the check for a TT in the xHCI virtual
device allocation to only check if the usb_device references a parent's
TT.

Make sure not to set the TT information on LS/FS devices directly
connected to the roothub.  The xHCI host doesn't really have a TT, and the
host will throw an error when those virtual device TT fields are set for a
device connected to the roothub.  We need this check because the xHCI
driver will shortly register two roothubs: a USB 2.0 roothub and a USB 3.0
roothub.

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