hwmon: (pmbus) Introduce the concept of "write-only" attributes

hwmon: (pmbus) Introduce the concept of "write-only" attributes

Attributes intended to clear sensor history are intended to be writeable
only. Reading those attributes today results in reporting more or less
random values. To avoid ABI surprises, have those attributes explicitly
return 0 when reading.

Fixes: 787c095edaa9d ("hwmon: (pmbus/core) Add support for rated attributes")
Reviewed-by: Sanman Pradhan <psanman@juniper.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>