net: dsa: lantiq_gswip: ignore MDIO buses disabled in OF

If the "lantiq,xrx200-mdio" child has status = "disabled", the MDIO bus
creation should be avoided. Use of_device_is_available() to check for
that, and take advantage of 2 facts:

- of_device_is_available(NULL) returns false
- of_node_put(NULL) is a no-op

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>