ACPI / hotplug: Rework deferred execution of acpi_device_hotplug()

Since the only function executed by acpi_hotplug_execute() is
acpi_device_hotplug() and it only is called by the ACPI core,
simplify its definition so that it only takes two arguments, the
ACPI device object pointer and event code, rename it to
acpi_hotplug_schedule() and move its header from acpi_bus.h to
the ACPI core's internal header file internal.h.  Modify the
definition of acpi_device_hotplug() so that its first argument is
an ACPI device object pointer and modify the definition of
struct acpi_hp_work accordingly.

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