In a heavy EMI environment, reading the current config from the MAX31865 can
fail, such as switching from a 4-wire sensor to a 3-wire sensor. This causes
the temperature value to be off wildly, but still technically valid, so it
doesn't get reported as a sensor failure.
Since we know what configuration we want, rather than send it to the MAX31865
on setup and ask for it over and over (propagating any error as we write it
back), instead store the base configuration and work from that to change modes.
This not only avoids propagating any error, it also saves a lot of unnecessary
reads from the MAX31865.
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