dt-bindings: net: mvusb: Fix example errors

The example for Marvell USB to MDIO Controller doesn't build:

Error: Documentation/devicetree/bindings/net/marvell,mvusb.example.dts:18.9-14 syntax error
FATAL ERROR: Unable to parse input tree

This is due to label refs being used which can't be resolved.

Fixes: 61e0150c

 ("dt-bindings: net: add marvell usb to mdio bindings")
Cc: Tobias Waldekranz <tobias@waldekranz.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>