fix calbration convergence condition

[hkwi]

The code looks intending seeking for a stable condition that all readings are within 2 difference. The readings are unsigned int, so the calculated difference will not be nagative, instead it will be very large unsigned int.

int main(int argc, char* argv){
  unsigned int a,b;
  a = 1;
  b = 2;
  printf("%u %u", a-b, b-a);
  // 4294967295 1
}