mv643xx_eth: fix unicast address filter corruption on mtu change

When mv643xx_eth_open() is called to up an interface, port_start()
will first re-program the unicast address filter, and then
re-initialise the PORT_CONFIG register, but that will disable unicast
promiscuous mode if it was enabled by the unicast address filter setup.

This isn't a problem on ifconfig up, as ->set_rx_mode() will be called
shortly afterwards which will program the filters again, but it does
trigger when changing the MTU, which calls mv643xx_eth_stop() and then
mv643xx_eth_open() by hand to repopulate the receive rings with skbuffs
of the new size.

Swap the initialisation of the PORT_START register and the call to
the unicast filter setup function to fix this.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>