Add missing usb_free_urb on failure path after usb_alloc_urb.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@km exists@
local idexpression e;
expression e1,e2,e3;
type T,T1;
identifier f;
@@

* e = usb_alloc_urb(...)
... when any
    when != e = e1
    when != e1 = (T)e
    when != e1(...,(T)e,...)
    when != &e->f
if(...) { ... when != e2(...,(T1)e,...)
                 when != e3 = e
                 when forall
(
             return <+...e...+>;
|
*             return ...;
) }
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Chris Ball <cjb@laptop.org>