rt2x00: Only initialize the minimum needed fields of PCI TX descriptors.

In preparation of replacing the statically allocated data DMA buffers with DMA-mapped
skb's we need to change the TXD handling of the PCI drivers, by moving the programming
of the buffer address fields to the actual TXD writing at TX time, instead of at start-up
time.

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