xhci: Fix ring leak in failure path of xhci_alloc_virt_device()

This is a stable-only fix for the backport of commit 5d9b70f7
("xhci: Don't add a virt_dev to the devs array before it's fully
allocated").

In branches that predate commit c5628a2a ("xhci: remove endpoint
ring cache") there is an additional failure path in
xhci_alloc_virt_device() where ring cache allocation fails, in
which case we need to free the ring allocated for endpoint 0.
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Cc: Mathias Nyman <mathias.nyman@intel.com>

drivers/usb/host/xhci-mem.c

@@ -1071,7 +1071,8 @@ int xhci_alloc_virt_device(struct xhci_hcd *xhci, int slot_id,
 
 	return 1;
 fail:
-
+	if (dev->eps[0].ring)
+		xhci_ring_free(xhci, dev->eps[0].ring);
 	if (dev->in_ctx)
 		xhci_free_container_ctx(xhci, dev->in_ctx);
 	if (dev->out_ctx)