USB: xhci: drop spinlock in xhci_urb_enqueue() error path.

Make sure the error path in xhci_urb_enqueue() releases the spinlock
before it returns.  Reported by Oliver in
	http://marc.info/?l=linux-usb&m=124091637311832&w=2

Reported-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/xhci-hcd.c

@@ -591,7 +591,8 @@ int xhci_urb_enqueue(struct usb_hcd *hcd, struct urb *urb, gfp_t mem_flags)
 	if (!xhci->devs || !xhci->devs[slot_id]) {
 		if (!in_interrupt())
 			dev_warn(&urb->dev->dev, "WARN: urb submitted for dev with no Slot ID\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto exit;
 	}
 	if (!test_bit(HCD_FLAG_HW_ACCESSIBLE, &hcd->flags)) {
 		if (!in_interrupt())