net: stmmac: use xpcs_create_mdiodev()

commit  fa6bc1bc0d6ca4bdf19ee20848a87fe3291a1a1d from
https://github.com/nxp-imx/linux-imx lf-6.1.y

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>
(cherry picked from commit 727e373f

)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>