platform/x86: ideapad-laptop: Fix kbd backlight not remembered among boots

platform/x86: ideapad-laptop: Fix kbd backlight not remembered among boots

On some models supported by ideapad-laptop, the HW/FW can remember the
state of keyboard backlight among boots. However, it is always turned
off while shutting down, as a side effect of the LED class device
unregistering sequence.

This is inconvenient for users who always prefer turning on the
keyboard backlight. Thus, set LED_RETAIN_AT_SHUTDOWN on the LED class
device so that the state of keyboard backlight gets remembered, which
also aligns with the behavior of manufacturer utilities on Windows.

Fixes: 503325f84bc0 ("platform/x86: ideapad-laptop: add keyboard backlight control support")
Cc: stable@vger.kernel.org
Signed-off-by: Rong Zhang <i@rong.moe>
Reviewed-by: Hans de Goede <hansg@kernel.org>
Link: https://lore.kernel.org/r/20250707163808.155876-3-i@rong.moe
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>