leds: netxbig: set led_classdev max_brightness

This patch sets the led_classdev max_brightness to the maximum level
value supported by hardware. This allows to get rid of the brightness
conversion operation (from software [0:LED_FULL] to hardware ranges) in
brightness_set().

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>