mac80211: Add an 802.11n definition

This patch adds a HT Capability (DSSS/CCK Mode in 40MHz BSS)
definition to ieee80211.h

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>