hwmon: (pmbus/core) Use s64 instead of long for calculations

Using s64 type, instead of long type, for internal calculations and for
the sysfs interface.

This allows 64-bit values to appear correctly on 32-bit kernels.
As wattage is reported in microwatts, monitoring a power supply over
2KW requires this.

Although it may seem unlikely to run a 32-bit kernel on such a large
machine, enterprise servers often include a BMC, and the BMC might be
running a 32-bit kernel.

Signed-off-by: Josh Lehan <krellan@google.com>
[groeck: Removed Change-Id and other tags, reformatted description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>