usb: cdns3: gadget: fix NULL pointer dereference in ep_queue

In the Linux kernel, the following vulnerability has been resolved:

usb: cdns3: gadget: fix NULL pointer dereference in ep_queue

When the gadget endpoint is disabled or not yet configured, the ep->desc

pointer can be NULL. This leads to a NULL pointer dereference when

__cdns3_gadget_ep_queue() is called, causing a kernel crash.

Add a check to return -ESHUTDOWN if ep->desc is NULL, which is the

standard return code for unconfigured endpoints.

This prevents potential crashes when ep_queue is called on endpoints

that are not ready.