hwmon: (pmbus) Detect if chip is write protected

If a chip is write protected, we can not change any limits, and we can
not clear status flags. This may be the reason why clearing status flags
is reported to not work for some chips. Detect the condition in the pmbus
core. If the chip is write protected, set limit attributes as read-only,
and set the flag indicating that the status flag should be ignored.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>