mac80211: rt2x00: fix of ht20/ht40 switching fix

r42348 uses a wrong register

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42349 3c298f89-4303-0410-b956-a3cf2f4a3e73