Clean up two-second interval check

There was some handling of millis() overflow that is not needed.
Standard unsigned subtraction and wraparound already works as expected,
so this extra check can be removed (it even hurts, since it introduces 2
seconds after a wraparound where no new data will be read, even if it
could otherwise happen).

Also, this prevents calling millis() a second time, since its value is
already known.

DHT.cpp

@@ -117,15 +117,11 @@ boolean DHT::read(bool force) {
   // Check if sensor was read less than two seconds ago and return early
   // to use last reading.
   uint32_t currenttime = millis();
-  if (currenttime < _lastreadtime) {
-    // ie there was a rollover
-    _lastreadtime = 0;
-  }
   if (!force && !_firstreading && ((currenttime - _lastreadtime) < 2000)) {
     return _lastresult; // return last correct measurement
   }
   _firstreading = false;
-  _lastreadtime = millis();
+  _lastreadtime = currenttime;
 
   // Reset 40 bits of received data to zero.
   data[0] = data[1] = data[2] = data[3] = data[4] = 0;