rt2x00: Use skbdesc fields for descriptor initialization

In rt2x00lib_write_tx_desc() the skb->data and skb->len fields
were incorrectly used. For USB drivers both of those values
contain invalid data (skb->data points to the device descriptor,
skb->len contains the frame _and_ descriptor length).

Instead of using the skbuffer fields we should use the skbdesc
fields which are correctly initialized and contain all the data
that we need.

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