Simplify bus configuration.

Previous bus configuration struct was meant for flexibility, but it
turned out that the only dimension that flexibility would go was a
serial number parameter. Let's rotate that configuration matrix by 90
degrees and just go a straightforward route of discriminating against
interface type.

Test: VTS
Bug: 135918744
Change-Id: I08967d0f78c998b0582958eb51bd387f9dbe15fe