mv643xx_eth: transmit multiqueue support

As all the infrastructure for multiple transmit queues already exists
in the driver, this patch is entirely trivial.

The individual transmit queues are still serialised by the driver's
per-port private spinlock, but that will disappear (i.e. be replaced
by the per-subqueue ->_xmit_lock) in a subsequent patch.

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