ACPI / dock: Simplify dock_init_hotplug() and dock_release_hotplug()

Make dock_init_hotplug() and dock_release_hotplug() slightly simpler
and move some checks in those functions to the code paths where they
are needed.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>