xhci: don't create endpoint debugfs entry before ring buffer is set.

Make sure xHC completes the configure endpoint command and xhci driver
sets the ring pointers correctly before we create the user readable
debugfs file.

In theory there was a small gap where a user could have read the
debugfs file and cause a NULL pointer dereference error as ring
pointer was not yet set, in practise we want this change to simplify
the upcoming streams debugfs support.

Fixes: 02b6fdc ("usb: xhci: Add debugfs interface for xHCI driver")
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20200918131752.16488-10-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.c

@@ -1918,8 +1918,6 @@ static int xhci_add_endpoint(struct usb_hcd *hcd, struct usb_device *udev,
 	ep_ctx = xhci_get_ep_ctx(xhci, virt_dev->in_ctx, ep_index);
 	trace_xhci_add_endpoint(ep_ctx);
 
-	xhci_debugfs_create_endpoint(xhci, virt_dev, ep_index);
-
 	xhci_dbg(xhci, "add ep 0x%x, slot id %d, new drop flags = %#x, new add flags = %#x\n",
 			(unsigned int) ep->desc.bEndpointAddress,
 			udev->slot_id,
@@ -2952,6 +2950,7 @@ static int xhci_check_bandwidth(struct usb_hcd *hcd, struct usb_device *udev)
 		xhci_check_bw_drop_ep_streams(xhci, virt_dev, i);
 		virt_dev->eps[i].ring = virt_dev->eps[i].new_ring;
 		virt_dev->eps[i].new_ring = NULL;
+		xhci_debugfs_create_endpoint(xhci, virt_dev, i);
 	}
 command_cleanup:
 	kfree(command->completion);