USB: ohci: set urb->hcpriv = NULL immediately, after free it

although we can not say it is surely a bug.
it is better to set urb->hcpriv = NULL, after finish calling
urb_free_priv.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>