leds: trigger: pattern: Switch to using the new API kobj_to_dev()

Switch to using the new API kobj_to_dev() to fix the below warnning:
./drivers/leds/trigger/ledtrig-pattern.c:336:60-61: WARNING opportunity
for kobj_to_dev()

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