mwl8k: mwl8k_txq_xmit() rework

Various mwl8k_txq_xmit changes:
- Extract the QoS field before adding the DMA header.
- Only write to tx->status once, and only after all the other
  descriptor fields have been set.
- Do all tx state manipulation under the tx spinlock.
- Remove the priv->inconfig check, as all transmit queues will
  be frozen during config cycles, so we won't ever be asked to
  transmit if a config cycle is running.
- Remove some more dead code.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>