hwmon: (it87) Fix PWM frequency display for chips with newer PWM control

On chips with newer PWM control, the PWM frequency divider is 256
instead of 128. Since the base PWM frequency remained the same, the actual
PWM frequency is half of what it used to be with the older PWM control
mechanism.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>