We don't have to do this since we started counting the composite devices out
and back in, and relying on the parent being set at a specific instance seems
fragile in real-world testing.
The I²C proxy specification is not always shared with all other devices as
I originally hoped, instead there are other legacy devices that use different
sets of I²C commands.
Un-share various bits of code to allow for additional I²C devices to be added.
No logic changes.
