Fix timeout calculation

[terrillmoore]

Timeout calculation has a signed compare problem.

Arduino-Temperature-Control-Library/DallasTemperature.cpp

Lines 372 to 373 in a02a6d3

	unsigned long now = millis();
	while(!isConversionAvailable(deviceAddress) && (millis() - delms < now));

millis() - delms is unsigned (because now is unsigned and both sides of the < are promoted). At a rollover interval, this can exit too soon. The correct calculation is:

(millis() - now) < (unsigned) delms

The result of millis() - now is always the right number of elapsed milliseconds, regardless of rollover.