leds: add LED_ON brightness as boolean value

Some devices do not handle the led brightness or simply don't
care about it. Conceptually said devices want to just switch on
or off the led. It is useless in this case to have a 255 range
of brightness, while just having an LED_ON and LED_OFF improves
the boolean meaning of the led status.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>