rt2x00: Cleanup write_tx_desc() arguments

Send the skb structure with write_tx_desc() and use
the skbdesc structure to read all information about
the frame. This saves several arguments in the function
definition and it is easier to send more information
later as well.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>