exynos4212 liblights: fix keypad lighting up when screen backlight auto-adjusts

clone of http://review.cyanogenmod.com/#/c/18372/

becomes relevant when these are applied:
smdk4210 kernel: cypress touchkey led_timeout functionality (1/2)
http://review.cyanogenmod.com/#/c/18357/
galaxys2/i777: configurable touchkey backlight timeout (2/2)
http://review.cyanogenmod.com/#/c/18431/

Change-Id: I3d4df6574cdd3c6317199a884ef7ac72cb4c71a3