USB: xhci: Don't touch xhci_td after it's freed.

On a successful transfer, urb->td is freed before the URB is ready to be
given back to the driver.  Don't touch urb->td after it's freed.  This bug
would have only shown up when xHCI debugging was turned on, and the freed
memory was quickly reused for something else.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>