ASoC: rt1015p: correct indentation

Use common enum instead of oneOf and correct indentation warning:
  realtek,rt1015p.yaml:18:7: [warning] wrong indentation: expected 4 but found 6 (indentation)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Tzung-Bi Shih <tzungbi@google.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210819101020.26368-2-krzysztof.kozlowski@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>