mv643xx_eth: various cleanups

mv643xx_eth: various cleanups

- Remove unused MV643XX_DEFAULT_[RT]X_QUEUE_SIZE definitions.
- Remove ETH_TARGET enum -- it isn't used anywhere in the driver,
  and isn't even valid for non-mv643xx chip models, as those use
  different MBUS target IDs.
- Clean up comment and control flow in mv643xx_eth_change_mtu().
- Use mp->dev instead of mp->mii.dev in mv643xx_eth_tx_timeout_task().
- Make mv643xx_eth_free_tx_descs() static.
- Remove overzealous NULL check in mv643xx_eth_start_xmit().
- Use symbolic NETDEV_TX_* constants in mv643xx_eth_start_xmit().

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Reviewed-by: Tzachi Perelstein <tzachi@marvell.com>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>