mac80211: Do not disconnect on invalid operating class

Some APs include a non global operating class in their extended channel
switch information element. In such a case, as the operating class is not
known, mac80211 would decide to disconnect.

However the specification states that the operating class needs to be
taken from Annex E, but it does not specify from which table it should be
taken, so it is valid for an AP to use a non global operating class.

To avoid possibly unneeded disconnection, in such a case ignore the
operating class and assume that the current band is used, and if the
resulting channel and band configuration is invalid disconnect.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>