ASoC: fsl-ssi: reorder and document fsl_ssi_private

Reorder all variables in struct fsl_ssi_private to have groups that make
sense together. The patch also updates the struct documentation.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-By: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>