leds: gpio trigger: simplifications from core changes

The trigger core learned error handling for the activate callback and
to handle device attributes. Also make use of the module_led_trigger()
helper to simplify trigger registration.

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>