Bluetooth: hci_serdev: Add protocol check in hci_uart_dequeue().

This will help to check the status of protocol while dequeuing an
skb packet. In some instaces we will end up kernel crash,
where proto close is called and we trying to dequeue an packet.

[  500.142902] [<ffffff80080f9ce4>] do_raw_spin_lock+0x1c/0xe0
[  500.148643] [<ffffff80088f1c7c>] _raw_spin_lock_irqsave+0x38/0x48
[  500.154917] [<ffffff8008780ce8>] skb_dequeue+0x28/0x84
[  500.160209] [<ffffff8000ad6f48>] 0xffffff8000ad6f48
[  500.165230] [<ffffff8000ad6610>] 0xffffff8000ad6610
[  500.170257] [<ffffff80080c7ce8>] process_one_work+0x238/0x3e4
[  500.176174] [<ffffff80080c8330>] worker_thread+0x2bc/0x3d4
[  500.181821] [<ffffff80080cdabc>] kthread+0x138/0x140
[  500.186945] [<ffffff80080844e0>] ret_from_fork+0x10/0x18

Signed-off-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/hci_serdev.c

@@ -57,9 +57,10 @@ static inline struct sk_buff *hci_uart_dequeue(struct hci_uart *hu)
 {
 	struct sk_buff *skb = hu->tx_skb;
 
-	if (!skb)
-		skb = hu->proto->dequeue(hu);
-	else
+	if (!skb) {
+		if (test_bit(HCI_UART_PROTO_READY, &hu->flags))
+			skb = hu->proto->dequeue(hu);
+	} else
 		hu->tx_skb = NULL;
 
 	return skb;