net: dsa: sja1105: use xpcs_create_mdiodev()

Use the new xpcs_create_mdiodev() creator, which simplifies the
creation and destruction of the mdio device associated with xpcs.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>