rt2x00: Rename driver write_tx_datadesc callback function.

Now that the {usb,pci} specific write_tx_data functions are no longer
present we can rename the write_tx_datadesc callback function back to
its old name.

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