hwmon: (amc6821) Add support for pwm1_mode attribute

AMC6821 supports configuring if a fan is DC or PWM controlled.
Add support for the pwm1_mode attribute to make it runtime configurable.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>