stmmac: Fix 'eth0: No PHY found' regression

This patch manages the case when you have an Ethernet MAC with
a "fixed link", and not connected to a normal MDIO-managed PHY device.

The test of phy_bus_name was not helpful because it was never affected
and replaced by the mdio test node.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>