init: Break FindPlatformDevice() into a helper function

We want to use the logic for FindPlatformDevice() in a future change
to look for devices that have USB parents. Break out a helper
function.

This change is intended to be a no-op and just a reorganization.

Bug: 316324155
Test: See boot devices still found
Change-Id: Ic5a57a89339c43dea1334644be9386637169c61c