rt2x00: rt2800pci: fix tx path by not accessing the skb after it was DMA mapped

rt2800pci used the callback write_tx_desc to write the tx descriptor but
also to update the txwi which is part of the dma mapped skb. Since the
memory was already DMA mapped _before_ the call to write_tx_desc the
device didn't get the txwi data at all or only sporadically.

The call order is basically as follows (from rt2x00queue.c):
1) write_tx_data
2) rt2x00queue_map_txskb
3) write_tx_desc

Hence, we shouldn't touch the skb in write_tx_desc anymore.

To fix this issue create a new rt2800pci_write_tx_data callback and use it
for updating the txwi _before_ the memory gets DMA mapped.

The tx descriptor is still written (as before) in write_tx_desc.

This patch allows basic TX on an rt305x soc device but I'm pretty sure
that it will fix pci based cards as well. I can associate just fine with
an AP now but I wasn't able to get a wpa secured connection working yet.

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>