The EFM32 uses the provided _SILICON_LABS_32B_SERIES_0 and
_SILICON_LABS_32B_SERIES_1 definitions to enable or disable certain
code. With the introduction of new MCUs, there is also the
_SILICON_LABS_32B_SERIES_2 definition.
This PR ensures that the defines are explicit, and that #else
statements don't target the wrong series.
The ADC_STATUS_SINGLEACT indicates that an operation is active, while
the ADC_STATUS_SINGLEDV indicates that data is valid.
This fixes ADC returning values of unfinished reads.
