net: stmmac: Don't search for phys if mdio node is defined.

If a dt mdio entry has been added least assume that we wont
search for phys attached. The DT and of_mdiobus_register already do
this. This stops DSA phys being found and phys created for them, as
this is handled by the DSA driver.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>