hwmon: (pmbus) Use 64bit math for DIRECT format values

Power values in the 100s of watt range can easily blow past
32bit math limits when processing everything in microwatts.

Use 64bit math instead to avoid these issues on common 32bit ARM
BMC platforms.

Fixes: 442aba78 ("hwmon: PMBus device driver")
Signed-off-by: Robert Lippert <rlippert@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>