mv643xx_eth: replace array of skbs awaiting transmit completion with a queue

Get rid of the skb pointer array that we currently use for transmit
reclaim, and replace it with an skb queue, to which skbuffs are appended
when they are passed to the xmit function, and removed from the front
and freed when we do transmit queue reclaim and hit a descriptor with
the 'owned by device' bit clear and 'last descriptor' bit set.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>