linux/thermal.h: rename KELVIN_TO_CELSIUS to DECI_KELVIN_TO_CELSIUS

The macros KELVIN_TO_CELSIUS and CELSIUS_TO_KELVIN actually convert
between deciKelvins and Celsius, so rename them to reflect that. While
at it, use a statement expression in DECI_KELVIN_TO_CELSIUS to prevent
expanding the argument multiple times and get rid of a few casts.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>