rt2x00: Request TXWI pointer from driver

The only reason why the write_tx_data callback function
is needed inside the driver, is because the location of
the TXWI descriptor is different on PCI and USB hardware.
Except for the beacon, where the TXWI is always at the
start of the SKB buffer.

In both cases the drivers write_tx_data function only
wrap around the function rt2800_write_txwi. Move write_tx_data
completely into the rt2800lib library, and add a callback
function to obtain the TXWI pointer.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>