mv643xx_eth: Do not modify struct netdev tx_queue_len

This driver erroneously zeros dev->tx_queue_len, since
mp->tx_ring_size has not yet been initialized.  Actually,
the driver shouldn't modify tx_queue_len at all and should
leave the value set by alloc_etherdev(), currently 1000.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>