Fixes ADC read error.

In the ADCConversionGroup definition, I had set SQR3 incorrectly.
In it one specifies, for a given sequence of conversions, which
channels will be converted in conversion N.

I had copied code from the ChibiOS/RT example and so my instructions
to SQR3 were to convert nothing for the first conversion in a
sequence of 1 conversion.  Therefore, it did not convert anything
during conversion #1.  If I had told it to do two conversions, it
then would have converted the channel in question.

As Rick Campbell taught me, the bug is rarely the thing you have
just been working on.