net: stmmac: Fixed link does not need MDIO Bus

When using fixed link we don't need the MDIO bus support.

Reported-by: Heiko Stuebner <heiko@sntech.de>
Reported-by: kernelci.org bot <bot@kernelci.org>
Fixes: d3e014ec

 ("net: stmmac: platform: Fix MDIO init for platforms without PHY")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Acked-by: Sriram Dash <Sriram.dash@samsung.com>
Tested-by: Patrice Chotard <patrice.chotard@st.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail> # Lamobo R1 (fixed-link + MDIO sub node for roboswitch).
Signed-off-by: David S. Miller <davem@davemloft.net>