mac80211: unify setup of iw htmode for mesh and adhoc

This also fixes mac80211_prepare_vif iw set channel in monitor or
mesh mode.

Signed-off-by: Santiago Piccinini <spiccinini@altermundi.net>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[daniel@makrotopia.org: fixed commit message]