leds: mt6323: cosmetic change: use helper variable

Use helper variable dev instead of always writing &pdev->dev.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: John Crispin <john@phrozen.org>
Cc: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>