usb: gadget: dummy_hcd: allow to free requests on disabled endpoints

ep->desc is set to NULL on endpoint disable. That means once an endpoint
is disabled it is not possible to free requests. In my target gadget I
first disable endpoints to make sure I have no requests on the fly and
then free frequests. On dummy I am leaking memory here.
Since I can't imagine a reason why it should be a bad thing, lets allow
to free requests on disabled endpoints. On removal of composite the ep0
request is removed so lets allow that here as well.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>