rt2x00: don't write past the end when writing short descriptors on rt61

The space allocated in the skb for a descriptor is only 24 bytes when
setting up beacons in rt61 so make sure we don't write to the descriptor
words beyond that and corrupt the beacon packet.
Signed-off-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt61pci.c

@@ -1531,9 +1531,11 @@ static void rt61pci_write_tx_desc(struct rt2x00_dev *rt2x00dev,
 	rt2x00_set_field32(&word, TXD_W5_WAITING_DMA_DONE_INT, 1);
 	rt2x00_desc_write(txd, 5, word);
 
-	rt2x00_desc_read(txd, 11, &word);
-	rt2x00_set_field32(&word, TXD_W11_BUFFER_LENGTH0, skbdesc->data_len);
-	rt2x00_desc_write(txd, 11, word);
+	if (skbdesc->desc_len > TXINFO_SIZE) {
+		rt2x00_desc_read(txd, 11, &word);
+		rt2x00_set_field32(&word, TXD_W11_BUFFER_LENGTH0, skbdesc->data_len);
+		rt2x00_desc_write(txd, 11, word);
+	}
 
 	rt2x00_desc_read(txd, 0, &word);
 	rt2x00_set_field32(&word, TXD_W0_OWNER_NIC, 1);