Implement the port enable/disable callbacks, which enable/disable the
SERDES interfaces, if applicable. This should save a bit of
power/heat.

We also need to enable SERDES on CPU and DSA ports, so keep the
existing call to the op, but make it conditional.
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>