HID: google: switch to devm when registering keyboard backlight LED

We can use devm to register keyboard backlight LED on hammer devices, this
will allow us to use HID's driver data for something else later.

Link: https://lore.kernel.org/r/20220228075446.466016-2-dmitry.torokhov@gmail.com
Tested-by: Stephen Boyd <swboyd@chromium.org> # coachz, wormdingler
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>