mac80211: implement Operating Mode Notification extended NSS support

Somehow we missed this for a long time, but similar to the extended
NSS support in VHT capabilities, we need to have this in Operating
Mode notification.

Implement it by
 * parsing the 160/80+80 bit there and setting the bandwidth
   appropriately
 * having callers of ieee80211_get_vht_max_nss() pass in the current
   max NSS value as received in the operating mode notification in
   order to modify it appropriately depending on the extended NSS
   bits.

This updates all drivers that use it, i.e. only iwlwifi/mvm.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200326150855.098483728cfa.I4e8c25d3288441759c2793247197229f0696a37d@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>