ACPI / scan: Indicate to platform when hot remove returns busy

In hotplug logic, it always indicates non-specific failure to
platform through _OST when handing ACPI hot-remove event failed. Then
platform terminates the hot-remove process but it can not identify
the reason.

Base on current hot-remove code, there have two situations that it
returns busy:

 - OSPM try to offline an individual device, but the device offline
   function returns "busy".

 - When the ejection event is applied to an "not offlined yet"
   container.  OSPM sends a kobject change event to userspace and
   returns "busy".

Both of them will returns -EBUSY to ACPI device hotplug function.
Then, the hotplug function indicates non-specific failure to platform
just like for any other error, e.g. -ENODEV or -EIO.

The benefit to the platform for identifying the OS "busy" state is
that it can use a different approach to handle the "busy" instead of
simply terminating the hot-remove operation for an unknown reason.
For example, the platform can wait for a while and then re-trigger
hot-remove.

Signed-off-by: "Lee, Chun-Yi" <jlee@suse.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[ rjw: Changelog massage ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>