mac80211: rt2x00 correctly set ht20/ht40 filter

Simple patch to correct ht20/ht40 switching. Tested with rt3290/rt3352/rt5350

Signed-off-by: Serge Vasilugin <vasilugin@yandex.ru>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42348 3c298f89-4303-0410-b956-a3cf2f4a3e73