wifi: mac80211: add back SPDX identifier

Looks like I lost that by accident, add it back.

Fixes: 076fc877 ("wifi: cfg80211: remove wdev mutex")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * BSS client mode implementation
  * Copyright 2003-2008, Jouni Malinen <j@w1.fi>