Settings: fix backlight illumination wrong summary

This fixes the visual indicator of the button illumination summary after
trying to disable. Previously after setting the timeout to never, would
display a summary of "1", or with 5, it would end up displaying 6 after
applying the settings.

Ticket: CYNGNOS-2565

Change-Id: Ia982e4849d0099b6e208b362770a431dc89dd2f1
Signed-off-by: Roman Birg <roman@cyngn.com>