hwmon: (pmbus) Expose PEC debugfs attribute

Enable runtime debug control of whether the PEC byte is exchanged with
the PMBus device.

Some manufacturers have asked for the PEC to be disabled as part of
debugging driver communication issues with devices.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200910021106.2958382-1-andrew@aj.id.au


[groeck: Replace %1llu with %llu]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>