hwmon: (max6650) Allow fan shutdown and initial rpm target

The fan can be stopped by writing "3" to pwm1_enable in sysfs.

Add devicetree property for early initialization of the fan controller
to prevent overheating, for example when resetting the board while the
fan was completely turned off.

Also improve error reporting, I2C failures were ignored while writing
new values.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>