esp8266/modules: Fix negative temperature in ds18x20 driver.

esp8266/modules/ds18x20.py

@@ -43,4 +43,7 @@ class DS18X20:
                 t = buf[0] >> 1
             return t - 0.25 + (buf[7] - buf[6]) / buf[7]
         else:
-            return (buf[1] << 8 | buf[0]) / 16
+            t = buf[1] << 8 | buf[0]
+            if t & 0x8000: # sign bit set
+                t = -((t ^ 0xffff) + 1)
+            return t / 16