MFC: r254005

- Fix a bug in the MSI allocation logic so an MSI is also employed if a
  controller supports only a single message. I haven't seen such an adapter
  out in the wild, though, so this change likely is a NOP.
  While at it, further simplify the MSI allocation logic; there's no need
  to check the number of available messages on our own as pci_alloc_msi(9)
  will just fail if it can't provide us with the single message we want.
- Nuke the unused softc of aacch(4).