usb: gadget: aspeed_udc: check endpoint DMA allocation

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: aspeed_udc: check endpoint DMA allocation ast_udc_probe() allocates a coherent DMA buffer used as the backing store for endpoint buffers. ast_udc_init_ep() derives per-endpoint buffer pointers from udc->ep0_buf, so a failed allocation is dereferenced during probe. Check the allocation before endpoint setup. The existing probe error path called ast_udc_remove(), which unregisters the gadget unconditionally and is not safe before usb_add_gadget_udc() succeeds. Add a local cleanup helper for probe failures so pre-registration failures only unwind the resources that were actually initialized. This was found by a local static analysis checker for unchecked allocator returns while scanning Linux 6.16. The change was checked by applying it to current mainline and by running checkpatch. I do not have access to Aspeed UDC hardware, so no runtime testing was performed.