The `adc_read()` code for STM32F0xx expects to get the 0-based channel
number in `mux.input`, but the `pinToMux()` code for STM32F0xx was
attempting to pass the CHSELR bit mask in that field, which resulted in
selecting a wrong channel, therefore `analogReadPin()` did not work
properly for the STM32F0xx chips. Fix `pinToMux()` to put the channel
number in that field (this matches the behavior for other supported
chips and also allows selection of channels 16...18, which can be used
to access the builtin temperature, reference voltage and VBAT sensors).
81ca83296f