ACPI / dock: Drop the hp_lock mutex from struct dock_station

The only existing user of the hp_lock mutex in struct dock_station,
hotplug_dock_devices(), is always called under acpi_scan_lock and
cannot race with another instance of itself, so drop the mutex
which is not necessary.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>