net: mdio-octeon: use platform_{get,set}_drvdata()

Use the wrapper functions for getting and setting the driver data
using platform_device instead of using dev_{get,set}_drvdata()
with &pdev->dev, so we can directly pass a struct platform_device.
This is a purely cosmetic change.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>