cfg80211: avoid freeing last_request while in flight

Avoid freeing the last request while it is being processed. This can
happen in some cases if reg_work is kicked for some reason while the
currently pending request is in flight.

Cc: Sander Eikelenboom <linux@eikelenboom.it>
Tested-by: Eliad Peller <eliad@wizery.com>
Tested-by: Colleen Twitty <colleen@cozybit.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>