net: axienet: replace mdiobus_write() with mdiodev_write()

Commit 0ebecb26 ("net: mdio: Add helper functions for accessing
MDIO devices") added support for mdiodev accessor operations that
neatly wrap the mdiobus accessor operations. Since we are dealing with
a mdio device here, update the driver to use mdiodev_write().

Tested-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>