staging: brcm80211: only allow one call to add_interface callback

The driver only supports one network interface to be associated
with it. Only for the first call it does actually something. For
subsequent calls it does return -ENODEV error code.
Reported-by: NJohannes Berg <johannes@sipsolutions.net>
Reviewed-by: NRoland Vossen <rvossen@broadcom.com>
Reviewed-by: NPieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: NFranky Lin <frankyl@broadcom.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/brcm80211/brcmsmac/mac80211_if.c

@@ -341,7 +341,10 @@ brcms_ops_add_interface(struct ieee80211_hw *hw, struct ieee80211_vif *vif)
 
 	wl = hw->priv;
 	LOCK(wl);
-	err = brcms_up(wl);
+	if (!wl->pub->up)
+		err = brcms_up(wl);
+	else
+		err = -ENODEV;
 	UNLOCK(wl);
 
 	if (err != 0)