net: dsa: start and stop the PHY state machine

dsa_slave_open() should start the PHY library state machine for its PHY
interface, and dsa_slave_close() should stop the PHY library state
machine accordingly.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>