ds18b20 temperature freezing wrong (BSP-419)

[morgana2313]

The temperature returned with this component in function ds18b20_get_temperature is wrong (4095C) for temperatures below 0C. The sign-bit is not extended properly because of the typecast to int16_t.

Fix:

-    *ret_temperature = (((int16_t)scratchpad.temp_msb << 8) | lsb_masked)  / 16.0f;
+    *ret_temperature = (((int8_t)scratchpad.temp_msb << 8) | lsb_masked)  / 16.0f;