rt2x00: Fix rt2800usb TX descriptor writing.

The recent changes to skb handling introduced a bug in the rt2800usb
TX descriptor writing whereby the length of the USB packet wasn't
calculated correctly.
Found via code inspection, as the devices themselves didn't seem to mind.
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>

drivers/net/wireless/rt2x00/rt2800usb.c

@@ -414,7 +414,7 @@ static void rt2800usb_write_tx_desc(struct rt2x00_dev *rt2x00dev,
 	 */
 	rt2x00_desc_read(txi, 0, &word);
 	rt2x00_set_field32(&word, TXINFO_W0_USB_DMA_TX_PKT_LEN,
-			   skb->len + TXWI_DESC_SIZE);
+			   skb->len - TXINFO_DESC_SIZE);
 	rt2x00_set_field32(&word, TXINFO_W0_WIV,
 			   !test_bit(ENTRY_TXD_ENCRYPT_IV, &txdesc->flags));
 	rt2x00_set_field32(&word, TXINFO_W0_QSEL, 2);