NFC: protect nci_data_exchange transactions

Protect 'cb' and 'cb_context' arguments in nci_data_exchange.
In fact, this implements a queue with max length of 1 data
exchange transactions in parallel.

Signed-off-by: Ilan Elias <ilane@ti.com>
Acked-by: Lauro Ramos Venancio <lauro.venancio@openbossa.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/nfc/nci/ntf.c

@@ -215,7 +215,7 @@ static void nci_rf_deactivate_ntf_packet(struct nci_dev *ndev,
 	}
 
 	/* complete the data exchange transaction, if exists */
-	if (ndev->data_exchange_cb)
+	if (test_bit(NCI_DATA_EXCHANGE, &ndev->flags))
 		nci_data_exchange_complete(ndev, NULL, -EIO);
 }