Remove debug leftovers that were causing memory corruption. Writes of 64 or more bytes from the USB host side caused memory corruption before this commit. I've tested it in a STM32L4 board and it is working as expected.
README
This example uses the USB port as a serial port and implements a loopback interface that immediately echos back whatever it receives. After flashing the evaluation board with the usbserial program and connecting it to a host computer with a USB cable, you can run the echo.py command as follows:
$ sudo ./echo.py hello world hello world $