net: dsa: use slave device phydev

There is no need to store a phy_device in dsa_slave_priv since
net_device already provides one. Simply s/p->phy/dev->phydev/.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>