hwmon: (acpi_power_meter) Cleanup and optimizations

An unsigned value can not be smaller than 0. Remove the check for it.
Use DIV_ROUND_CLOSEST for divide operations converting milli-degrees C into
degrees C. Limit maximum accepted trip point temperature to INT_MAX.

This patch fixes Coverity #115214: Unsigned compared against 0

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>