ar71xx: fix WD My Net N750 switch led cfg

Now that the switch is working correctly I had the chance to actually
test the LED config.

Signed-off-by: Felix Kaechele <heffer@fedoraproject.org>
Patchwork: http://patchwork.openwrt.org/patch/4616/


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

SVN-Revision: 39129