Extends struct usb_endpoint_descriptor to make it possible to provide
the USB host with class-specific extensions to endpoint descriptors.
The approach taken, based on extra and extralen and removing the sizeof()
from USB_DT_ENDPOINT_SIZE, is identical to the approach used to add
class-specific extensions to interface descriptors. All libopencm3-examples
use the USB_DT_ENDPOINT_SIZE (rather than directly using sizeof) so there
should be no compatibility problems resulting from this change.
d3aa579e0a