mac80211: check csa wiphy flag in ibss before switching

When external CSA IEs are received (beacons or action messages), a
channel switch is triggered as well. This should only be allowed on
devices which actually support channel switches, otherwise disconnect.
(For the corresponding userspace invocation, the wiphy flag is checked
in nl80211).

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>