usb: gadget: Store endpoint pointer in usb_request

Gadget function drivers often have goto-based error handling in their
bind paths, which can be bug-prone. Refactoring these paths to use
__free() scope-based cleanup is desirable, but currently blocked.

The blocker is that usb_ep_free_request(ep, req) requires two
parameters, while the __free() mechanism can only pass a pointer to the
request itself.

Store an endpoint pointer in the struct usb_request. The pointer is
populated centrally in usb_ep_alloc_request() on every successful
allocation, making the request object self-contained.

Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://lore.kernel.org/r/20250916-ready-v1-1-4997bf277548@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250916-ready-v1-1-4997bf277548@google.com