hwmon: (pmbus) Use long variables for register to data conversions

Using integer variable types for register to data conversions can cause
overflows especially for power calculations, which are in microwatt.
Use long variables instead.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org # 2.6.39+