Add and use the following functions:
	mv643xx_eth_port_enable_tx()
	mv643xx_eth_port_enable_rx()
	mv643xx_eth_port_disable_tx()
	mv643xx_eth_port_disable_rx()

so that ports are enabled/disabled consistently.
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>