This is basically a cut down version of MAX31855 without reference
temperature reading and lower resolution (only positive, 12bit only).
This implements 16bit protocol (31855 uses 32bit). SetOption94 enables
the new behavior.
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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Changes:
v2:
* treat occasional 0xfff as an error
* do not add new sensor pins, use SetOption94 instead