cfg80211/mac80211: make ieee80211_send_layer2_update a public function

mainline inclusion
from mainline-v4.20-rc1
commit 30ca1aa5
category: bugfix
bugzilla: 13690
CVE: CVE-2019-5108

This patch is prepare for fixing 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>
Conflicts:
  net/wireless/util.c
[yyl: adjust context]

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Wenan Mao <maowenan@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>