leds: core: fix misleading comment after workqueue removal from drivers

Now that workqueues have been removed from individual drivers and
were replaced with a core-internal workqueue we shouldn't
encourage people to add new workqueues to drivers.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>