leds: oneshot trigger: simplifications from core changes

The trigger core learned error handling for the activate callback and
can handle device attributes now. This allows simplifying the driver
considerably. Note that .deactivate() is only called when .activate()
succeeded, so the check for .activated can go away in .deactivate().

Also make use of module_led_trigger() and do some minor coding style
improvements.

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>