Use set/get drv data in phydev's mdio device instead. Phy device priv
field maybe used by the external phy driver and should not be
overwritten.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.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>