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>