cfg80211/mac80211: make ieee80211_send_layer2_update a public function

cfg80211/mac80211: make ieee80211_send_layer2_update a public function

CVE-2019-5108

Make ieee80211_send_layer2_update() a common function so other drivers
can re-use it.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
(backported from commit 30ca1aa536211f5ac3de0173513a7a99a98a97f3)
[ Connor Kuehl: context adjustments ]
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>