rt2x00: Split rt2x00lib_write_tx_desc()

Split rt2x00lib_write_tx_desc() up into a TX descriptor initializor
and TX descriptor writer.

This split is required to properly allow mac80211 to move its
tx_control structure into the skb->cb array.
The rt2x00queue_create_tx_descriptor() function will read all tx control
information and convert it into a rt2x00 TX descriptor information structure.
After that function is complete, we have all information we needed from the
tx control structure and are free to start writing into the skb->cb array
for our own purposes.
rt2x00queue_write_tx_descriptor() will be in charge of really sending
the TX descriptor to the hardware and kicking the TX queue.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/rt2x00/rt2500usb.c b/drivers/net/wireless/rt2x00/rt2500usb.c
index 3be4c7e..6fe713a 100644
--- a/drivers/net/wireless/rt2x00/rt2500usb.c
+++ b/drivers/net/wireless/rt2x00/rt2500usb.c
@@ -1676,6 +1676,7 @@
 	struct rt2x00_intf *intf = vif_to_intf(control->vif);
 	struct queue_entry_priv_usb_bcn *priv_bcn;
 	struct skb_frame_desc *skbdesc;
+	struct txentry_desc txdesc;
 	int pipe = usb_sndbulkpipe(usb_dev, 1);
 	int length;
 	u16 reg;
@@ -1686,6 +1687,14 @@
 	priv_bcn = intf->beacon->priv_data;
 
 	/*
+	 * Copy all TX descriptor information into txdesc,
+	 * after that we are free to use the skb->cb array
+	 * for our information.
+	 */
+	intf->beacon->skb = skb;
+	rt2x00queue_create_tx_descriptor(intf->beacon, &txdesc, control);
+
+	/*
 	 * Add the descriptor in front of the skb.
 	 */
 	skb_push(skb, intf->beacon->queue->desc_size);
@@ -1713,7 +1722,7 @@
 	rt2x00_set_field16(&reg, TXRX_CSR19_BEACON_GEN, 0);
 	rt2500usb_register_write(rt2x00dev, TXRX_CSR19, reg);
 
-	rt2x00lib_write_tx_desc(rt2x00dev, skb, control);
+	rt2x00queue_write_tx_descriptor(intf->beacon, &txdesc);
 
 	/*
 	 * USB devices cannot blindly pass the skb->len as the