net: mvneta: speed down the PHY, if WoL used, to save energy

Some PHYs connected to this ethernet hardware support the WoL feature.
But when WoL is enabled and the machine is powered off, the PHY remains
waiting for a magic packet at max speed (i.e. 1Gbps), which is a waste of
energy.

Slow down the PHY speed before stopping the ethernet if WoL is enabled,
and save some energy while the machine is powered off or sleeping.

Tested using an Armada 370 based board (LS421DE) equipped with a Marvell
88E1518 PHY.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>