ar71xx: fix LED config for DIR-869 A1

ar71xx: fix LED config for DIR-869 A1

This fixes the LED configuration for the D-Link DIR-869 A1. In order to
support the device I probed around using an initramfs image for the
UniFi AC. Pulling GPIO 15 to low enabled the LEDs while high disabled them.
GPIO 16 set to low meant that the color was white while pulling it to high
made the color change to orange. The past code was written based upon these
findings.
However, running a flashed image I now discovered that GPIO 15 controls the
orange LEDs while GPIO 16 controls the white ones and that both are active
when low. This means that the GPIOs were inverted and one active_low was set
wrong which this patch fixes.

Behavior of the LED front after this patch is applied:

cat /sys/devices/platform/leds-gpio/leds/d-link:white:status/brightness
0   -> white LEDs are OFF
255 -> white LEDs are ON

cat /sys/devices/platform/leds-gpio/leds/d-link:orange:status/brightness
0   -> orange LEDs are OFF
255 -> orange LEDs are ON

If the brightness of both is set to 255 the LED front will be white.
If the brightness of both is set to 0 the LED front will be off.

Signed-off-by: Florian Beier <beier.florian@gmail.com>