mac80211: disconnect iface if CSA unexpectedly fails

It doesn't make much sense to leave a crippled
interface running.

As a side effect this will unblock tx queues with
CSA reason immediately after failure instead of
until after userspace requests interface to stop.

This also gives userspace an opportunity to
indirectly see CSA failure.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
[small code cleanup]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>