Ensure that the data in ringbuff is accessed in the right order

This fixes a potential issue with concurrency and the compiler being allowed to reorder instructions in the ringbuff (The whole topic is quite interesting and caused quite some discussions in our working group). Unfortunately this causes msp430-gcc 4.7.0 to produce worse code (for some reason the optimizer does not detect, that a register isn't needed anymore and does not reuse it, therefore needing an additional instruction). Nonetheless the side effects caused by this in the future may become quite ugly.