How to increase UART speed?

I'm developing for this RZ/A2M board but the UART speed is too slow, only 115200 baud. I tried increasing it in the code, but I just get garbage because I can't figure out how to change the speed of the USB to serial adapter on the board. I tried "stty -F /dev/ttyACM0 230400" but it can not be set to a speed higher than 115200. 

The data sheet says that chip supports up to 4 Mbps serial speeds


How can I set it faster?