hwmon: (pmbus) Mark lowest/average/highest/rated attributes as read-only

hwmon: (pmbus) Mark lowest/average/highest/rated attributes as read-only

Writing those attributes is not supported, so mark them as read-only.

Prior to this change, attempts to write into these attributes returned
an error.

Mark boolean fields in struct pmbus_limit_attr and in struct
pmbus_sensor_attr as bit fields to reduce configuration data size.
The data is scanned only while probing, so performance is not a concern.

Fixes: 6f183d33a02e6 ("hwmon: (pmbus) Add support for peak attributes")
Reviewed-by: Sanman Pradhan <psanman@juniper.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>