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>