ASoC: fsl_ssi: We do support master mode now

Since commit aafa85e7

 (ASoC: fsl_ssi: Add DAI master mode support for SSI on
i.MX series) master mode is supported, so update the comments and code to
reflect that.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>