Callers (acpi_memhotplug.c, dock.c and others) check for the return
value of acpi_bus_add() and assume a valid device was returned in
case zero was returned.

Thus return -ENODEV if no device was found in acpi_bus_scan and
propagate this through acpi_bus_add and acpi_bus_start.

Also remove a confusing comment in acpiphp_glue.c, acpi_bus_scan
will and cannot invoke if acpi_bus_add returns no valid device.
Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>