led: triggers: Initialize LED_INIT_DEFAULT_TRIGGER if trigger is brought after class

Trigger driver can be initialized after the LED class device driver.  In
such case led_trigger_set_default() won't be called and flag
LED_INIT_DEFAULT_TRIGGER should be set from led_trigger_register().

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>