mac80211: move some HT code out of mlme.c

Some of the HT code in mlme.c is misplaced:
 * constants/definitions belong to the ieee80211.h header
 * code being used in other modes as well shouldn't be there

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>