Fix negative temperature in ds18x20 driver

[ernitron]

As per discussion on this forum thread http://forum.micropython.org/viewtopic.php?f=16&t=2564#p15232
I think there is a bug in Onewire ds18x20 driver with negative temperatures.

The proposed fix is just for Dallas temperature sensors of 0x28 class. I have no possibility to check for 0x10 but I guess there is a similar issue with negative values.

Fix has been tested and provide correct results for negative temperatures. Code was derived from similar code working for Arduino.

[dpgeorge]

Thanks! Merged in e5f0655 with a few tweaks: removed the return -(t/16) line and changed the t=... line before it to include the negative; changed 16.0 to 16 because that's more efficient in the bytecode and produces the same result.

I couldn't test it but I assume it works.

I have no possibility to check for 0x10 but I guess there is a similar issue with negative values.

No, I think that version of the device is ok, it seems to have a check for negative numbers already.