[libata] ACPI: Properly handle bay devices in dock stations

* Differentiate between bay devices in dock stations and others:

 - When an ACPI_NOTIFY_EJECT_REQUEST appears, just signal uevent to
   userspace (that is when the optional eject button on a bay device is
   pressed/pulled) giving the possibility to unmount file systems and to
   clean up. Also, only send uevent in case we get an EJECT_REQUEST
   without doing anything else. In other cases, you'll get an add/remove
   event because libata attaches/detaches the device.

 - In case of a dock event, which in turn signals an
   ACPI_NOTIFY_EJECT_REQUEST, immediately detach the device, because it
   may already have been gone

* In case of an ACPI_NOTIFY_DEVICE/BUS_CHECK, evaluate _STA to check if
  the device has been plugged or unplugged. If plugged, hotplug it, if
  unplugged, just signal event to userspace
  (initial patch by Matthew Garrett <mjg59@srcf.ucam.org>)

* Call ACPI _EJ0 for detached devices

Signed-off-by: Holger Macht <hmacht@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>