backend/omnia: determine max brightness from sysfs
Upstream kernel did not yet apply the patch that sets the max_brightness sysfs property of Omnia LEDs to 1 (from 255).
Because of this, when using upstream kernel we need to set brightness to 255. Otherwise the LEDs are very dim, because although we set multi_intensity to "255 255 255", this is then recomputed to "1 1 1" before kernel sends it to hardware, since brightness is set to 1/255.