acpi/pcib: If bus does not present, give other bus drivers chance

acpi/pcib: If bus does not present, give other bus drivers chance

- Call acpi_DeviceIsPresent() during probing, so that if bus does not
  present other bus drivers could have chance to take over the bus.
- In acpi_pcib_attach(), assert that bus is present, since the presence
  of the bus is checked during probing.

Reported-by: swildner@