dt-bindings: net: ethernet-controller: drop redundant type from label

dtschema defines label as string, so $ref in other bindings is
redundant.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>