UPSTREAM: mac80211: add stations after AP start on reconfig

UPSTREAM: mac80211: add stations after AP start on reconfig

BUG=chrome-os-partner:10785
TEST=Manual (run wifi test suites)

When performing a HW restart for an AP mode interface, add stations back
only after the AP is beaconing. This mimics the normal flow of STA
addition on AP.

Some devices (wlcore) do not support adding stations before beaconing,
so this has the added benefit of making recovery work for them.

Change-Id: I087759324047648092e717ec7fe266e9b15d1776
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/30881
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Tan Gao <tgao@chromium.org>
Tested-by: Tan Gao <tgao@chromium.org>