xhci: Limit roothub ports to 15 USB3 & 31 USB2 ports.

The USB core allocates a USB 2.0 roothub descriptor that has room for 31
(USB_MAXCHILDREN) ports' worth of DeviceRemovable and PortPwrCtrlMask
fields.  Limit the number of USB 2.0 roothub ports accordingly.  I don't
expect to run into this limitation ever, but this prevents a buffer
overflow issue in the roothub descriptor filling code.

Similarly, a USB 3.0 hub can only have 15 downstream ports, so limit the
USB 3.0 roothub to 15 USB 3.0 ports.

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