altos/stmf0: Have fast ADC ring buffer code use wrap-around

Instead of requiring that the whole set of returned values fit
precisely in the ring, allow for wrap-around so that we can fetch an
odd number of ADC values. The previous version required that the fetch
amount always be a factor of the ADC buffer size.

Signed-off-by: Keith Packard <keithp@keithp.com>