ar71xx: swconfig: use the local helper to set LED brightness on link down

Use the 'swconfig_trig_set_brightness' function to set
the brightness value of the LED when the link goes down.
This ensures that the last brighness value is saved into
a local variable which is used to track the actual LED
status.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39402