xhci: Change xhci_find_slot_id_by_port() API.

xhci_find_slot_id_by_port() tries to map the port index to the slot ID for
the USB device.  In the future, there will be two xHCI roothubs, and their
port indices will overlap.  Therefore, xhci_find_slot_id_by_port() will
need to use information in the roothub's usb_hcd structure to map the port
index and roothub speed to the right slot ID.

Add a new parameter to xhci_find_slot_id_by_port(), in order to pass in
the roothub's usb_hcd structure.

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