leds: Fix led-gpio active_low default brightness

When gpio_direction_output() is called, led_dat->active_low is used
as default value. This means that the led will always be off by
default. cdev.brightness should really have been set to LED_OFF
unconditionally to reflect this behavior.

Signed-off-by: Raphael Assenat <raph@8d.com>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>