netdev/phy: skip disabled mdio-mux nodes

The mdio-mux driver scans all child mdio nodes, without regard to whether
the node is actually used.  Some device trees include all possible
mdio-mux nodes and rely on the boot loader to disable those that are not
present, based on some run-time configuration.  Those nodes need to be
skipped.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>