Is the `adc_clear_flag` function broken?

I think the clearing should be:

```c
ADC_SR(ADC1) &= ~ADC_SR_JEOC; 
```

Instead of:

```c
ADC_SR(ADC1) = ~ADC_SR_JEOC; 
```

If it is broken, then it should probably be fixed in `libopencm3` too.