b43: work around a locking issue in ->set_tim()

ops->set_tim() must be atomic, so b43 trying to acquire a mutex leads
to a kernel crash. This patch trades an easy to trigger crash in AP
mode for an unlikely race condition. According to Michael, the real
fix would be to allow set_tim() to sleep, since b43 is not the only
driver that needs to sleep in all callbacks.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/main.c

@@ -4521,9 +4521,8 @@ static int b43_op_beacon_set_tim(struct ieee80211_hw *hw,
 {
 	struct b43_wl *wl = hw_to_b43_wl(hw);
 
-	mutex_lock(&wl->mutex);
+	/* FIXME: add locking */
 	b43_update_templates(wl);
-	mutex_unlock(&wl->mutex);
 
 	return 0;
 }