cfg80211/mac80211: make ieee80211_send_layer2_update a public function

commit 30ca1aa536211f5ac3de0173513a7a99a98a97f3 upstream.

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>
[bwh: Backported to 4.14 as dependency of commit 3e493173b784
 "mac80211: Do not send Layer 2 Update frame before authorization"]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>