staging: brcm80211: removed compile warning

A warning was generated if CONFIG_BRCMDBG=n. Fixed this by introducing an
function.
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/brcm80211/brcmsmac/wl_mac80211.c

@@ -183,10 +183,11 @@ static int wl_ops_start(struct ieee80211_hw *hw)
 
 static void wl_ops_stop(struct ieee80211_hw *hw)
 {
+#ifdef BRCMDBG
 	struct wl_info *wl = hw->priv;
 	ASSERT(wl);
+#endif /*BRCMDBG*/
 	ieee80211_stop_queues(hw);
-	return;
 }
 
 static int