mac80211: rt2x00: enable setting wifi led polarity from OF

This patch enables setting the wifi led polarity from OF.
It can be used in a DTS' wmac section like this:

   ralink,led-polarity = <1>;

Valid values are 0 or 1.

Signed-off-by: Felix Kaechele <heffer@fedoraproject.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37607 3c298f89-4303-0410-b956-a3cf2f4a3e73