leds: lm3533: Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
drivers/leds/leds-lm3533.c:611:60-61: WARNING opportunity for kobj_to_dev().

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>