Too fast sampling can lock up board

**[Original report](https://bitbucket.org/abe_k/pterodaq/issue/42) by kevin_karplus (Bitbucket: [kevin_karplus](https://bitbucket.org/kevin_karplus), ).**

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If the sampling rate (either timed or from pin changes) is so fast that the USB stack can't get any packets out, then the board has to be power cycled to be reset.
