net: stmmac: 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: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>