ACPI: Make acpi_bus_add() and acpi_bus_start() visibly different

The current ACPI namespace scanning code suggests that acpi_bus_add()
and acpi_bus_start() share some code.  In fact, however, they are
completely different code paths (except for the initial checks), so
refactor the code to make that distinction visibly clear.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Toshi Kani <toshi.kani@hp.com>