platform/x86: thinkpad_acpi: Add LED_RETAIN_AT_SHUTDOWN to led_class_devs

Add the LED_RETAIN_AT_SHUTDOWN flag to the registered led_class_devs so
that the LEDs do not get turned-off when reloading the driver and thus so
that they also stay under default EC control when reloading the driver,
unless explicitly overridden by the user.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20211123210524.266705-1-hdegoede@redhat.com