fix Thermocouple readings

Author: leonardocavagnis

Diff for: src/utility/THERMOCOUPLE/MAX31855.h

@@ -41,7 +41,7 @@ class MAX31855Class {
 private:
   uint32_t readSensor();
   float    _coldOffset;
-  int _cs;
+  PinName _cs;
   SPIClass* _spi;
   SPISettings _spiSettings;