leds: ktd2692: Remove work queue

Now the core implements the work queue, remove it from the drivers,
and switch to using brightness_set_blocking op.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Ingi Kim <ingi2.kim@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>