struct usb_endpoint_descriptor * usb_epnum_to_ep_desc (struct usb_device * dev, unsigned epnum);
This walks the device descriptor for the currently active configuration, and returns a pointer to the endpoint with that particular endpoint number, or null.
Note that interface descriptors are not required to assign endpont numbers sequentially, so that it would be incorrect to assume that the first endpoint in that descriptor corresponds to interface zero. This routine helps device drivers avoid such mistakes.