broadcom-wl: align with mac80211 config

 - Support HT40 instead of HT40+/HT40- like mac80211
 - Enable 11n if htmode is HT20 or HT40

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43063 3c298f89-4303-0410-b956-a3cf2f4a3e73