ACPI: handle re-enumeration, when acpi_devices might already exist

acpi_bus_scan() traverses the namespace to enumerate devices and uses
acpi_add_single_object() to create acpi_devices.  When the platform
notifies us of a hot-plug event, we need to traverse part of the namespace
again to figure out what appeared or disappeared.  (We don't yet call
acpi_bus_scan() during hot-plug, but I plan to do that in the future.)

This patch makes acpi_add_single_object() notice when we already have
an acpi_device, so we don't need to make a new one.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>