Modify USB code to adapt to the latest API in libopencm3

Change in the code corresponding to my change in libopencm3.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Add basic support for DFU_DNLOAD request

This commit add support for simple uploading of the firmware form the
PC to the microcontroller. This code lacks CRC calculation and is
still far from usable, but it loads the data into the flash memory and
successfully verifies it.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>