at76c50x-usb: additional disconnect fixes

Additional attempts to fix Oops on disconnect, that appear to be successful.
However, some may be extraneous.

The cancel_delayed_work call is probably the most necessary.  The
device_unplugged check may not be necessary.  del_timer_sync may not
be necessary either, but the Oops I was receiving was related to
timers.  Hence the addition.
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/at76c50x-usb.c

@@ -1497,6 +1497,9 @@ static void at76_work_set_promisc(struct work_struct *work)
 					      work_set_promisc);
 	int ret = 0;
 
+	if (priv->device_unplugged)
+		return;
+
 	mutex_lock(&priv->mtx);
 
 	priv->mib_buf.type = MIB_LOCAL;
@@ -2290,6 +2293,7 @@ static void at76_delete_device(struct at76_priv *priv)
 	tasklet_kill(&priv->rx_tasklet);
 
 	if (priv->mac80211_registered) {
+		cancel_delayed_work(&priv->dwork_hw_scan);
 		flush_workqueue(priv->hw->workqueue);
 		ieee80211_unregister_hw(priv->hw);
 	}
@@ -2307,6 +2311,8 @@ static void at76_delete_device(struct at76_priv *priv)
 
 	kfree(priv->bulk_out_buffer);
 
+	del_timer_sync(&ledtrig_tx_timer);
+
 	if (priv->rx_skb)
 		kfree_skb(priv->rx_skb);