net: mvmdio: defer probe of orion-mdio if a clock is not ready

Defer probing of the orion-mdio interface when getting a clock returns
EPROBE_DEFER. This avoids locking up the Armada 8k SoC when mdio is used
before all clocks have been enabled.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>