Update architecture switch to match Arduino convention

[4-20ma]

Refer to https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#working-with-multiple-architectures.

Use either:

#if defined(ARDUINO_ARCH_AVR)
  // AVR-specific code
#elif defined(ARDUINO_ARCH_SAM)
  // SAM-specific code
#else
  // generic, non-platform specific code
#endif

or:

#if defined(ARDUINO_ARCH_AVR)
  // AVR-specific code
#elif defined(ARDUINO_ARCH_SAM)
  // SAM-specific code
#else
  #error “This library only supports boards with an AVR or SAM processor.”
#endif