hwmon: (npcm-750-pwm-fan) Change initial pwm target to 255

Change initial PWM target to 255 to prevent overheating, for example
when BMC hangs in userspace or when userspace fan control application is
not implemented yet.

Signed-off-by: Kun Yi <kunyi@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>