net: enetc: use lynx_pcs_create_mdiodev()

Use the newly introduced lynx_pcs_create_mdiodev() which simplifies the
creation and destruction of the lynx PCS.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>