mac80211: move ieee80211_enable_ht function to mlme.c

It really belongs into that file since it is only relevant
for managed mode. Move 1:1, not even whitespace changes,
but make it static and remove from header file.

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