ASoC: davinci-mcasp: Index ruledata in drvdata with substream->stream

The serializer direction definitions runs from 1 to 2, which does not
suite the purpose. The substream->stream is perfect for the purpose
and should have been used from the beginning.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>