leds: triggers: handle .trigger_data and .activated() in the core

This helps keeping these two fields consistent and drivers don't need to
care for this themselves any more.

Note that .activated isn't set to true automatically because that might
confuse some triggers when deactivating (e.g. ledtrig-gpio).

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>