device/kobo/sysfs_light: Explicitly handle warmth/brightness = 0
device/kobo/sysfs_light: Explicitly handle warmth/brightness = 0

For warmth and/or brightness = 0, we need to make sure that the
red/green/white LEDs are actually turned off, regardless of the values
for gain/offset.

lgeek/koreader,mihailim/koreader,pazos/koreader,poire-z/koreader,Hzj-jie/koreader,Frenzie/koreader,NiLuJe/koreader,houqp/koreader,koreader/koreader,poire-z/koreader,koreader/koreader,Markismus/koreader,mwoz123/koreader,NiLuJe/koreader,Frenzie/koreader