fujitsu-laptop: Rework brightness of eco led

For the sake of internal consistency, unset maximum brightness of eco
led and make it activatable only on values >= LED_FULL.

Signed-off-by: Matej Groma <matejgroma@gmail.com>
Acked-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>