hwmon: (acpi_power_meter) Fix unintentional integer overflow

Expression with two integer variables is calculated as integer before it is
converted to u64. This may result in an integer overflow. Fix by declaring
trip point variables as s64 instead of int.

This patch addresses Coverity #200596: Unintentional integer overflow.

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