usb: fallback safely when a configuration descriptor is too large

When a USB configuration descriptor was larger than our USB buffer
(512 bytes), we were skipping the full descriptor reading but then we
were still parsing and using it, triggering memory corruptions.
Now in that case, it just skips this device enumeration and displays the
appropriate message to the user, so he can fix the buffer if he wants.

This bug was triggered by some UVC webcams which have very large
configuration descriptors (e.g. a couple of kB) describing all their
supported video encodings.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>