mv643xx_eth: allow multiple TX queues

As with the multiple RX queue support, allow the platform code to
specify that the hardware we are running on supports multiple TX
queues.  This patch only uses the highest-numbered enabled queue
to send packets to for now, this can be extended later to enable
QoS and such.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>