ACPI: Update _OST handling for notify

When the kernel calls _OSC with OSC_SB_HOTPLUG_OST_SUPPORT bit
set at boot-time, the OS is responsible for calling _OST for
ACPI hotplug events.  However, when hotplug.enabled attribute
is unset for ACPI scan drivers, their notify handlers are removed
and _OST is not called for ACPI hotplug events as a result.

This patch keeps the notify handler of ACPI scan drivers,
acpi_hotplug_notify_cb(), installed regardless of the state of
hotplug.enabled.  The notify handler then checks if hotplug.enabled
is set for the associated scan handler.  If unset, the notify
handler calls _OST with a proper error code.  The patch also
eliminates ACPI namespace walk when hotplug.enabled is changed
via sysfs.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>