mac80211: handle width changes from opmode notification IE in beacon

An AP can send an operating channel width change in a beacon
opmode notification IE as long as there's a change in the nss as
well (See 802.11ac-2013 section 10.41).
So don't limit updating to nss only from an opmode notification IE.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>