ACPI / hotplug / PCI: Rework namespace scanning and trimming routines

The acpiphp_bus_trim() and acpiphp_bus_add() functions need not
return error codes that are never checked, so redefine them and
simplify them a bit.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>