leds: backlight trigger: simplifications from core changes

Use the new module_led_trigger() helper. Also use
attribute support from the trigger core. Drop error message on
allocation failure as kzalloc() already screams loudly when failing. Use
wrappers to get and set trigger data.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>