ACPI / dock: Make 'docked' sysfs attribute work as documented

After recent ACPI core changes acpi_bus_get_device() will always
succeed for dock station ACPI device objects, so show_docked()
should not use that function's return value as an indicator of
whether or not the dock device is present.

Make it use acpi_device_enumerated() for this purpose.

Fixes: 202317a5 (ACPI / scan: Add acpi_device objects for all device nodes in the namespace)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>