net: phy: mdio-octeon: use platform_set_drvdata()

Use the wrapper functions for getting and setting the driver data using
platform_device instead of using dev_set_drvdata() with &pdev->dev,
so we can directly pass a struct platform_device.

Signed-off-by: Libo Chen <libo.chen@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>