hwmon: (amc6821) Convert to with_info API

Convert to use with_info API to simplify the code and make it easier
to maintain. This also reduces code size by approximately 20%.

No functional change intended.
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>