net: dsa: Avoid null pointer when failing to connect to PHY

When phylink_of_phy_connect fails, dsa_slave_phy_setup tries to save the
day by connecting to an alternative PHY, none other than a PHY on the
switch's internal MDIO bus, at an address equal to the port's index.

However this does not take into consideration the scenario when the
switch that failed to probe an external PHY does not have an internal
MDIO bus at all.

Fixes: aab9c406 ("net: dsa: Plug in PHYLINK support")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>