When the battery is replaced for the RTC, the oscillator is stopped.
Additionally, existing code recognizes the mcp7941x RTC, but this RTC has an
enable bit at the same location as the "Clock Halt" bit on the ds1307, with an
opposite assertion (set == on, whereas CH set == clock stopped). Thus the
current code halts the clock, with no way to enable it.
This fixes both conditions, by properly handling the differences in the CH/ST
bits, and by attempting to enable the clock when stopped.