Thermostats v2.2 and below have a noisy and offset feedback current
`tec_i` caused by missing hardware on 2 MAX1968 TEC driver pins:
1. A missing RC filter on the ITEC pin that would have isolated CPU
sampling pulses from the signal; and
2. Some missing buffering on the VREF pin that would have avoided
loading the VREF signal, preventing voltage drops from the nominal 1.5V.
Since the resulting signal `tec_i` derived from these two signals can
have an error of around +/- 100mA, and readback may affect the stability
performance of the Thermostat, disable current readback entirely on
affected hardware revisions for now.
See https://github.com/sinara-hw/Thermostat/issues/117 and
https://github.com/sinara-hw/Thermostat/issues/120.
On hardware revisions v3.x and above, this would be fixed.
atse