mac80211: adding 802.11n configuration flows

This patch configures the 802.11n mode of operation
internally in ieee80211_conf structure and in the low-level
driver as well (through op conf_ht).
It does not include AP configuration flows.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>