[BUG] pxa27x_udc: possible recursive locking detected in pxa_ep_queue

[David Brownell]

> As a side note, from a very quick glance at pxa25x_udc and omap_udc, I'm a bit
> surprised they didn't suffer from the same behaviour.

Haven't touched either of those drivers in years.  But a really
quick glance shows pxa25x_udc::done() setting a "stopped" flag,
and omap_udc::done() doing the same, before calling out to the
completion callback.

- Dave