Add rate control options and some 11n features to net80211 rate control code. (r255024)
	1) Add some rc options chip can provide to rc code when it init.
	2) Implement some 11n features. rc code can get the corresponding feature capability and decide whether to use it. If some 11n features are choosen to be used, rc code must fill corresponding flags. Other flags that rc code may not be interested by now are filled by ieee80211_ratectl_complete_rcflags.