backlight: lp855x: Make sure props struct is zeroed

The driver occasionally got stuck in suspend mode or other strange
states as those parts of the props struct were never initialized.

Signed-off-by: Werner Johansson <werner.johansson@sonymobile.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>