dt-bindings: iio: adc: 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>
Link: https://lore.kernel.org/r/20240226123004.91061-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>