dell-laptop: touchpad LED should persist its status after S3

Touchpad LED will not turn on after S3, it will make the touchpad status
doesn't consist with the LED.
By adding one flag to let the LED device restore it's status.

Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>