usb: gadget: dummy_hcd: don't assign ->desc on error case

If the stream check fails then we leave ep->desc assigend but we return
with an error code. The caller assumes the endpoint is not enabled
(which is the case) but it can not enable it again due to this
assigment.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>