rt2x00: Split of TXWI writing to write_tx_data callback in rt2800usb.

Align with the way PCI devices are handled, even though it is not
strictly necessary.

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>