hwmon: (pmbus/core) improve handling of write protected regulators

hwmon: (pmbus/core) improve handling of write protected regulators

Writing PMBus protected registers does succeed from the smbus perspective,
even if the write is ignored by the device and a communication fault is
raised. This fault will silently be caught and cleared by pmbus irq if one
has been registered.

This means that the regulator call may return succeed although the
operation was ignored.

With this change, the operation which are not supported will be properly
flagged as such and the regulator framework won't even try to execute them.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
[groeck: Adjust to EXPORT_SYMBOL_NS_GPL API change]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>