net: stmmac: fix invalid call to mdiobus_get_phy()

In a number of cases the driver assigns a default value of -1 to
priv->plat->phy_addr. This may result in calling mdiobus_get_phy()
with addr parameter being -1. Therefore check for this scenario and
bail out before calling mdiobus_get_phy().

Fixes: 42e87024

 ("net: stmmac: Fix case when PHY handle is not present")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/669f9671-ecd1-a41b-2727-7b73e3003985@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>