leds: lm3692: Use exponential brightness mode

This makes brightness sliders use the full range.

This was lingering since Carlsbad since i wanted to move the properties into DT.

I appreciate any testing. Since we switch from linear to exponential the phone may come up with a very dim screen if initial brightness was very low. At least in my tests it was bright enough to fix this via the shell's volume slider so i'd not do anything special here.

Closes: #118 (closed)