UPSTREAM: cfg80211: export interface stopping function

Upstream commit f04c22033c25f71617ac62bcfe75698baa17a0b8.

This exports a new cfg80211_stop_iface() function.

This is intended for driver internal interface
combination management and channel switching.

Due to locking issues (it re-enters driver) the
call is asynchronous and uses cfg80211 event
list/worker.

BUG=N/A
TEST=AVL test with Core7 iwl7000 driver

Change-Id: Iffc285665f05483a8bf9c57bd1450b24d2173d43
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/209985
Reviewed-by: Paul Stewart <pstew@chromium.org>