net: mvpp2: ensure the port is forced down while changing modes

Ensure that the port is forced down while reconfiguring, controlling
this via mac_prepare() and mac_finish() so that it is down while we
are configuring our (future) PCS.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>