leds: led-class: Retain the latest user brightness request

Retain the latest user brightness request. This ensures
that when a max brightness limit is altered, the last
brightness request is taken into account, when restoring
the current brightness.

Change-Id: I6461e06f64abe336cabc27a3992d734f1fb745c6
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>