usb: dfu: check requested length (wLength) during DFU_UPLOAD

During DFU_UPLOAD, the host could requests more data
as stated in wTransferSize. Limit upload length to the
size of the request buffer (USB_REQUEST_BUFFER_SIZE).

Signed-off-by: Johann Fischer <j.fischer@phytec.de>
Signed-off-by: David Brown <david.brown@linaro.org>