dm: usb: xhci: Use a function to get xhci_ctrl

Rather than getting this directly from struct usb_device, call a function
to obtain it. This will make it possible for driver model to provide it
another way.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index 0951e87..1f48933 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1259,4 +1259,6 @@
 int xhci_mem_init(struct xhci_ctrl *ctrl, struct xhci_hccr *hccr,
 		  struct xhci_hcor *hcor);
 
+struct xhci_ctrl *xhci_get_ctrl(struct usb_device *udev);
+
 #endif /* HOST_XHCI_H_ */