rt2x00: add txdesc parameter to write_tx_data

Extend the write_tx_data callback with a txdesc parameter to allow
access to the tx desciptor while preparing the tx data.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>