drivers: sensors: ti_hdc20xx: update temperature conversion formula

TI recently released a new version of the HDC2080 datasheet, which
slightly update the temperature conversion formula, with a temperature
offset of -40.5°C instead of -40°C. Adjust the code accordingly.

In addition the datasheet also describes a voltage dependent
compensation of -0.08°C/V above 1.8V, however it's not something easily
doable with the current sensor framework, so just ignore that part.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>