rsi: fix null pointer dereference during rsi_shutdown()

Appearently the hw pointer can be NULL while the module is loaded and
in that case rsi_shutdown() crashes due to the unconditional dereference.
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/rsi/rsi_91x_sdio.c

@@ -1468,12 +1468,15 @@ static void rsi_shutdown(struct device *dev)
 	struct rsi_91x_sdiodev *sdev =
 		(struct rsi_91x_sdiodev *)adapter->rsi_dev;
 	struct ieee80211_hw *hw = adapter->hw;
-	struct cfg80211_wowlan *wowlan = hw->wiphy->wowlan_config;
 
 	rsi_dbg(ERR_ZONE, "SDIO Bus shutdown =====>\n");
 
+	if (hw) {
+		struct cfg80211_wowlan *wowlan = hw->wiphy->wowlan_config;
+
 		if (rsi_config_wowlan(adapter, wowlan))
 			rsi_dbg(ERR_ZONE, "Failed to configure WoWLAN\n");
+	}
 
 	if (IS_ENABLED(CONFIG_RSI_COEX) && adapter->priv->coex_mode > 1 &&
 	    adapter->priv->bt_adapter) {