net: stmmac: trigger PCS EEE to turn off on link down

The current implementation enable PCS EEE feature in the event of link
up, but PCS EEE feature is not disabled on link down.

This patch makes sure PCE EEE feature is disabled on link down.

Fixes: 656ed8b0

 ("net: stmmac: fix EEE init issue when paired with EEE capable PHYs")
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>