cfg80211: export interface stopping function

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.

CRs-Fixed: 1114710
Change-Id: I2784dc34b6a20fbe8697473e37cacf8edd35deca
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


Git-commit: f04c2203
[rsirasan@codeaurora.org: Resolved merge conflicts]
Signed-off-by: Rajeev Kumar Sirasanagandla <rsirasan@codeaurora.org>