nl80211: send event when AP operation is stopped

There are a few cases, e.g. suspend, where an AP interface is
stopped by the kernel rather than by userspace request, most
commonly when suspending. To let userspace know about this,
send the NL80211_CMD_STOP_AP command as an event every time
an AP interface is stopped. This also happens when userspace
did in fact request the AP stop, but that's not a problem.

For full-MAC drivers this may need to be extended to also
cover cases where the device stopped the AP operation for
some reason, this a bit more complicated because then all
cfg80211 state also needs to be reset; such API is not part
of this patch.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>