hwmon: (amc6821) Stop accepting invalid pwm values

The pwm value range is well defined from 0..255. Don't accept any values
outside this range.

This changes the valid range of pwm1_auto_point2_pwm from 0..254 to 0..255,
meaning it can now be equivalent to not only pwm1_auto_point1_pwm (which is
always 0) but also to pwm1_auto_point3_pwm (which is always 255). While
that may not be practical, there seems to be no technical reason for
preventing a user from doing it.

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